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POSTCONF(5)                   File Formats Manual                  POSTCONF(5)



NAME
       postconf - Postfix configuration parameters

SYNOPSIS
       postconf parameter ...

       postconf -e "parameter=value" ...

DESCRIPTION
       The Postfix main.cf configuration file specifies parameters that
       control the operation of the Postfix mail system. Typically the file
       contains only a small subset of all parameters; parameters not
       specified are left at their default values.

       The general format of the main.cf file is as follows:

       ⊕      Each logical line has the form "parameter = value".  Whitespace
              around the "=" is ignored, as is whitespace at the end of a
              logical line.

       ⊕      Empty lines and whitespace-only lines are ignored, as are lines
              whose first non-whitespace character is a `#'.

       ⊕      A logical line starts with non-whitespace text. A line that
              starts with whitespace continues a logical line.

       ⊕      A parameter value may refer to other parameters.

              ⊕      The expressions "$name" and "${name}" are recursively
                     replaced with the value of the named parameter. The
                     parameter name must contain only characters from the set
                     [a-zA-Z0-9_]. An undefined parameter value is replaced
                     with the empty value.

              ⊕      The expressions "${name?value}" and "${name?{value}}" are
                     replaced with "value" when "$name" is non-empty. The
                     parameter name must contain only characters from the set
                     [a-zA-Z0-9_]. These forms are supported with Postfix
                     versions >= 2.2 and >= 3.0, respectively.

              ⊕      The expressions "${name:value}" and "${name:{value}}" are
                     replaced with "value" when "$name" is empty. The
                     parameter name must contain only characters from the set
                     [a-zA-Z0-9_]. These forms are supported with Postfix
                     versions >= 2.2 and >= 3.0, respectively.

              ⊕      The expression "${name?{value1}:{value2}}" is replaced
                     with "value1" when "$name" is non-empty, and with
                     "value2" when "$name" is empty.  The "{}" is required for
                     "value1", optional for "value2". The parameter name must
                     contain only characters from the set [a-zA-Z0-9_].  This
                     form is supported with Postfix versions >= 3.0.

              ⊕      The first item inside "${...}" may be a relational
                     expression of the form: "{value3} == {value4}". Besides
                     the "==" (equality) operator Postfix supports "!="
                     (inequality), "<", "<=", ">=", and ">". The comparison is
                     numerical when both operands are all digits, otherwise
                     the comparison is lexicographical. These forms are
                     supported with Postfix versions >= 3.0.

              ⊕      Each "value" is subject to recursive named parameter and
                     relational expression evaluation, except where noted.

              ⊕      Whitespace before or after each "{value}" is ignored.

              ⊕      Specify "$$" to produce a single "$" character.

              ⊕      The legacy form "$(...)" is equivalent to the preferred
                     form "${...}".

       ⊕      When the same parameter is defined multiple times, only the last
              instance is remembered.

       ⊕      Otherwise, the order of main.cf parameter definitions does not
              matter.

       The remainder of this document is a description of all Postfix
       configuration parameters. Default values are shown after the parameter
       name in parentheses, and can be looked up with the "postconf -d"
       command.

       Note: this is not an invitation to make changes to Postfix
       configuration parameters. Unnecessary changes can impair the operation
       of the mail system.

2bounce_notice_recipient (default: postmaster)
       The recipient of undeliverable mail that cannot be returned to the
       sender.  This feature is enabled with the notify_classes parameter.

access_map_defer_code (default: 450)
       The numerical Postfix SMTP server response code for an access(5) map
       "defer" action, including "defer_if_permit" or "defer_if_reject". Prior
       to Postfix 2.6, the response is hard-coded as "450".

       Do not change this unless you have a complete understanding of RFC
       5321.

       This feature is available in Postfix 2.6 and later.

access_map_reject_code (default: 554)
       The numerical Postfix SMTP server response code for an access(5) map
       "reject" action.

       Do not change this unless you have a complete understanding of RFC
       5321.

address_verify_cache_cleanup_interval (default: 12h)
       The amount of time between verify(8) address verification database
       cleanup runs. This feature requires that the database supports the
       "delete" and "sequence" operators.  Specify a zero interval to disable
       database cleanup.

       After each database cleanup run, the verify(8) daemon logs the number
       of entries that were retained and dropped. A cleanup run is logged as
       "partial" when the daemon terminates early after "postfix reload",
       "postfix stop", or no requests for $max_idle seconds.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is h (hours).

       This feature is available in Postfix 2.7.

address_verify_default_transport (default: $default_transport)
       Overrides the default_transport parameter setting for address
       verification probes.

       This feature is available in Postfix 2.1 and later.

address_verify_local_transport (default: $local_transport)
       Overrides the local_transport parameter setting for address
       verification probes.

       This feature is available in Postfix 2.1 and later.

address_verify_map (default: see postconf -d output)
       Lookup table for persistent address verification status storage.  The
       table is maintained by the verify(8) service, and is opened before the
       process releases privileges.

       The lookup table is persistent by default (Postfix 2.7 and later).
       Specify an empty table name to keep the information in volatile memory
       which is lost after "postfix reload" or "postfix stop". This is the
       default with Postfix version 2.6 and earlier.

       Specify a location in a file system that will not fill up. If the
       database becomes corrupted, the world comes to an end. To recover,
       delete (NOT: truncate) the file and do "postfix reload".

       Postfix daemon processes do not use root privileges when opening this
       file (Postfix 2.5 and later).  The file must therefore be stored under
       a Postfix-owned directory such as the data_directory.  As a migration
       aid, an attempt to open the file under a non-Postfix directory is
       redirected to the Postfix-owned data_directory, and a warning is
       logged.

       Examples:

       address_verify_map = hash:/var/db/postfix/verify
       address_verify_map = btree:/var/db/postfix/verify

       This feature is available in Postfix 2.1 and later.

address_verify_negative_cache (default: yes)
       Enable caching of failed address verification probe results.  When this
       feature is enabled, the cache may pollute quickly with garbage.  When
       this feature is disabled, Postfix will generate an address probe for
       every lookup.

       This feature is available in Postfix 2.1 and later.

address_verify_negative_expire_time (default: 3d)
       The time after which a failed probe expires from the address
       verification cache.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.1 and later.

address_verify_negative_refresh_time (default: 3h)
       The time after which a failed address verification probe needs to be
       refreshed.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is h (hours).

       This feature is available in Postfix 2.1 and later.

address_verify_pending_request_limit (default: see postconf -d output)
       A safety limit that prevents address verification requests from
       overwhelming the Postfix queue. By default, the number of pending
       requests is limited to 1/4 of the active queue maximum size
       (qmgr_message_active_limit). The queue manager enforces the limit by
       tempfailing requests that exceed the limit. This affects only unknown
       addresses and inactive addresses that have expired, because the
       verify(8) daemon automatically refreshes an active address before it
       expires.

       This feature is available in Postfix 3.1 and later.

address_verify_poll_count (default: normal: 3, overload: 1)
       How many times to query the verify(8) service for the completion of an
       address verification request in progress.

       By default, the Postfix SMTP server polls the verify(8) service up to
       three times under non-overload conditions, and only once when under
       overload.  With Postfix version 2.5 and earlier, the SMTP server always
       polls the verify(8) service up to three times by default.

       Specify 1 to implement a crude form of greylisting, that is, always
       defer the first delivery request for a new address.

       Examples:

       # Postfix <= 2.6 default
       address_verify_poll_count = 3
       # Poor man's greylisting
       address_verify_poll_count = 1

       This feature is available in Postfix 2.1 and later.

address_verify_poll_delay (default: 3s)
       The delay between queries for the completion of an address verification
       request in progress.

       The default polling delay is 3 seconds.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

address_verify_positive_expire_time (default: 31d)
       The time after which a successful probe expires from the address
       verification cache.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.1 and later.

address_verify_positive_refresh_time (default: 7d)
       The time after which a successful address verification probe needs to
       be refreshed.  The address verification status is not updated when the
       probe fails (optimistic caching).

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.1 and later.

address_verify_relay_transport (default: $relay_transport)
       Overrides the relay_transport parameter setting for address
       verification probes.

       This feature is available in Postfix 2.1 and later.

address_verify_relayhost (default: $relayhost)
       Overrides the relayhost parameter setting for address verification
       probes. This information can be overruled with the transport(5) table.

       This feature is available in Postfix 2.1 and later.

address_verify_sender (default: $double_bounce_sender)
       The sender address to use in address verification probes; prior to
       Postfix 2.5 the default was "postmaster". To avoid problems with
       address probes that are sent in response to address probes, the Postfix
       SMTP server excludes the probe sender address from all SMTPD access
       blocks.

       Specify an empty value (address_verify_sender =) or <> if you want to
       use the null sender address. Beware, some sites reject mail from <>,
       even though RFCs require that such addresses be accepted.

       Examples:

       address_verify_sender = <>
       address_verify_sender = postmaster@my.domain

       This feature is available in Postfix 2.1 and later.

address_verify_sender_dependent_default_transport_maps (default:
       $sender_dependent_default_transport_maps)
       Overrides the sender_dependent_default_transport_maps parameter setting
       for address verification probes.

       This feature is available in Postfix 2.7 and later.

address_verify_sender_dependent_relayhost_maps (default:
       $sender_dependent_relayhost_maps)
       Overrides the sender_dependent_relayhost_maps parameter setting for
       address verification probes.

       This feature is available in Postfix 2.3 and later.

address_verify_sender_ttl (default: 0s)
       The time between changes in the time-dependent portion of address
       verification probe sender addresses. The time-dependent portion is
       appended to the localpart of the address specified with the
       address_verify_sender parameter. This feature is ignored when the probe
       sender addresses is the null sender, i.e. the address_verify_sender
       value is empty or <>.

       Historically, the probe sender address was fixed. This has caused such
       addresses to end up on spammer mailing lists, and has resulted in
       wasted network and processing resources.

       To enable time-dependent probe sender addresses, specify a non-zero
       time value. Specify a value of at least several hours, to avoid
       problems with senders that use greylisting. Avoid nice TTL values, to
       make the result less predictable.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.9 and later.

address_verify_service_name (default: verify)
       The name of the verify(8) address verification service. This service
       maintains the status of sender and/or recipient address verification
       probes, and generates probes on request by other Postfix processes.

address_verify_transport_maps (default: $transport_maps)
       Overrides the transport_maps parameter setting for address verification
       probes.

       This feature is available in Postfix 2.1 and later.

address_verify_virtual_transport (default: $virtual_transport)
       Overrides the virtual_transport parameter setting for address
       verification probes.

       This feature is available in Postfix 2.1 and later.

alias_database (default: see postconf -d output)
       The alias databases for local(8) delivery that are updated with
       "newaliases" or with "sendmail -bi".

       This is a separate configuration parameter because not all the tables
       specified with $alias_maps have to be local files.

       Examples:

       alias_database = hash:/etc/aliases
       alias_database = hash:/etc/mail/aliases

alias_maps (default: see postconf -d output)
       The alias databases that are used for local(8) delivery. See aliases(5)
       for syntax details.  Specify zero or more "type:name" lookup tables,
       separated by whitespace or comma. Tables will be searched in the
       specified order until a match is found.  Note: these lookups are
       recursive.

       The default list is system dependent.  On systems with NIS, the default
       is to search the local alias database, then the NIS alias database.

       If you change the alias database, run "postalias /etc/aliases" (or
       wherever your system stores the mail alias file), or simply run
       "newaliases" to build the necessary DBM or DB file.

       The local(8) delivery agent disallows regular expression substitution
       of $1 etc. in alias_maps, because that would open a security hole.

       The local(8) delivery agent will silently ignore requests to use the
       proxymap(8) server within alias_maps. Instead it will open the table
       directly. Before Postfix version 2.2, the local(8) delivery agent will
       terminate with a fatal error.

       Examples:

       alias_maps = hash:/etc/aliases, nis:mail.aliases
       alias_maps = hash:/etc/aliases

allow_mail_to_commands (default: alias, forward)
       Restrict local(8) mail delivery to external commands.  The default is
       to disallow delivery to "|command" in :include:  files (see aliases(5)
       for the text that defines this terminology).

       Specify zero or more of: alias, forward or include, in order to allow
       commands in aliases(5), .forward files or in :include:  files,
       respectively.

       Example:

       allow_mail_to_commands = alias,forward,include

allow_mail_to_files (default: alias, forward)
       Restrict local(8) mail delivery to external files. The default is to
       disallow "/file/name" destinations in :include:  files (see aliases(5)
       for the text that defines this terminology).

       Specify zero or more of: alias, forward or include, in order to allow
       "/file/name" destinations in aliases(5), .forward files and in
       :include:  files, respectively.

       Example:

       allow_mail_to_files = alias,forward,include

allow_min_user (default: no)
       Allow a sender or recipient address to have `-' as the first character.
       By default, this is not allowed, to avoid accidents with software that
       passes email addresses via the command line. Such software would not be
       able to distinguish a malicious address from a bona fide command-line
       option. Although this can be prevented by inserting a "--" option
       terminator into the command line, this is difficult to enforce
       consistently and globally.

       As of Postfix version 2.5, this feature is implemented by
       trivial-rewrite(8).  With earlier versions this feature was implemented
       by qmgr(8) and was limited to recipient addresses only.

allow_percent_hack (default: yes)
       Enable the rewriting of the form "user%domain" to "user@domain".  This
       is enabled by default.

       Note: as of Postfix version 2.2, message header address rewriting
       happens only when one of the following conditions is true:

       ⊕      The message is received with the Postfix sendmail(1) command,

       ⊕      The message is received from a network client that matches
              $local_header_rewrite_clients,

       ⊕      The message is received from the network, and the
              remote_header_rewrite_domain parameter specifies a non-empty
              value.

       To get the behavior before Postfix version 2.2, specify
       "local_header_rewrite_clients = static:all".

       Example:

       allow_percent_hack = no

allow_untrusted_routing (default: no)
       Forward mail with sender-specified routing (user[@%!]remote[@%!]site)
       from untrusted clients to destinations matching $relay_domains.

       By default, this feature is turned off.  This closes a nasty open relay
       loophole where a backup MX host can be tricked into forwarding junk
       mail to a primary MX host which then spams it out to the world.

       This parameter also controls if non-local addresses with
       sender-specified routing can match Postfix access tables. By default,
       such addresses cannot match Postfix access tables, because the address
       is ambiguous.

alternate_config_directories (default: empty)
       A list of non-default Postfix configuration directories that may be
       specified with "-c config_directory" on the command line (in the case
       of sendmail(1), with the "-C" option), or via the MAIL_CONFIG
       environment parameter.

       This list must be specified in the default Postfix main.cf file, and
       will be used by set-gid Postfix commands such as postqueue(1) and
       postdrop(1).

       Specify absolute pathnames, separated by comma or space. Note: $name
       expansion is not supported.

always_add_missing_headers (default: no)
       Always add (Resent-) From:, To:, Date: or Message-ID: headers when not
       present.  Postfix 2.6 and later add these headers only when clients
       match the local_header_rewrite_clients parameter setting.  Earlier
       Postfix versions always add these headers; this may break DKIM
       signatures that cover non-existent headers.  The
       undisclosed_recipients_header parameter setting determines whether a
       To: header will be added.

always_bcc (default: empty)
       Optional address that receives a "blind carbon copy" of each message
       that is received by the Postfix mail system.

       Note: with Postfix 2.3 and later the BCC address is added as if it was
       specified with NOTIFY=NONE. The sender will not be notified when the
       BCC address is undeliverable, as long as all down-stream software
       implements RFC 3461.

       Note: with Postfix 2.2 and earlier the sender will be notified when the
       BCC address is undeliverable.

       Note: automatic BCC recipients are produced only for new mail.  To
       avoid mailer loops, automatic BCC recipients are not generated after
       Postfix forwards mail internally, or after Postfix generates mail
       itself.

anvil_rate_time_unit (default: 60s)
       The time unit over which client connection rates and other rates are
       calculated.

       This feature is implemented by the anvil(8) service which is available
       in Postfix version 2.2 and later.

       The default interval is relatively short. Because of the high frequency
       of updates, the anvil(8) server uses volatile memory only. Thus,
       information is lost whenever the process terminates.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

anvil_status_update_time (default: 600s)
       How frequently the anvil(8) connection and rate limiting server logs
       peak usage information.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.2 and later.

append_at_myorigin (default: yes)
       With locally submitted mail, append the string "@$myorigin" to mail
       addresses without domain information. With remotely submitted mail,
       append the string "@$remote_header_rewrite_domain" instead.

       Note 1: this feature is enabled by default and must not be turned off.
       Postfix does not support domain-less addresses.

       Note 2: with Postfix version 2.2, message header address rewriting
       happens only when one of the following conditions is true:

       ⊕      The message is received with the Postfix sendmail(1) command,

       ⊕      The message is received from a network client that matches
              $local_header_rewrite_clients,

       ⊕      The message is received from the network, and the
              remote_header_rewrite_domain parameter specifies a non-empty
              value.

       To get the behavior before Postfix version 2.2, specify
       "local_header_rewrite_clients = static:all".

append_dot_mydomain (default: Postfix >= 3.0: no, Postfix < 3.0: yes)
       With locally submitted mail, append the string ".$mydomain" to
       addresses that have no ".domain" information. With remotely submitted
       mail, append the string ".$remote_header_rewrite_domain" instead.

       Note 1: this feature is enabled by default. If disabled, users will not
       be able to send mail to "user@partialdomainname" but will have to
       specify full domain names instead.

       Note 2: with Postfix version 2.2, message header address rewriting
       happens only when one of the following conditions is true:

       ⊕      The message is received with the Postfix sendmail(1) command,

       ⊕      The message is received from a network client that matches
              $local_header_rewrite_clients,

       ⊕      The message is received from the network, and the
              remote_header_rewrite_domain parameter specifies a non-empty
              value.

       To get the behavior before Postfix version 2.2, specify
       "local_header_rewrite_clients = static:all".

application_event_drain_time (default: 100s)
       How long the postkick(1) command waits for a request to enter the
       Postfix daemon process input buffer before giving up.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

authorized_flush_users (default: static:anyone)
       List of users who are authorized to flush the queue.

       By default, all users are allowed to flush the queue.  Access is always
       granted if the invoking user is the super-user or the $mail_owner user.
       Otherwise, the real UID of the process is looked up in the system
       password file, and access is granted only if the corresponding login
       name is on the access list.  The username "unknown" is used for
       processes whose real UID is not found in the password file.

       Specify a list of user names, "/file/name" or "type:table" patterns,
       separated by commas and/or whitespace. The list is matched left to
       right, and the search stops on the first match. A "/file/name" pattern
       is replaced by its contents; a "type:table" lookup table is matched
       when a name matches a lookup key (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude a name from the list. The form "!/file/name" is
       supported only in Postfix version 2.4 and later.

       This feature is available in Postfix 2.2 and later.

authorized_mailq_users (default: static:anyone)
       List of users who are authorized to view the queue.

       By default, all users are allowed to view the queue.  Access is always
       granted if the invoking user is the super-user or the $mail_owner user.
       Otherwise, the real UID of the process is looked up in the system
       password file, and access is granted only if the corresponding login
       name is on the access list.  The username "unknown" is used for
       processes whose real UID is not found in the password file.

       Specify a list of user names, "/file/name" or "type:table" patterns,
       separated by commas and/or whitespace. The list is matched left to
       right, and the search stops on the first match. A "/file/name" pattern
       is replaced by its contents; a "type:table" lookup table is matched
       when a name matches a lookup key (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude a user name from the list. The form "!/file/name"
       is supported only in Postfix version 2.4 and later.

       This feature is available in Postfix 2.2 and later.

authorized_submit_users (default: static:anyone)
       List of users who are authorized to submit mail with the sendmail(1)
       command (and with the privileged postdrop(1) helper command).

       By default, all users are allowed to submit mail.  Otherwise, the real
       UID of the process is looked up in the system password file, and access
       is granted only if the corresponding login name is on the access list.
       The username "unknown" is used for processes whose real UID is not
       found in the password file. To deny mail submission access to all users
       specify an empty list.

       Specify a list of user names, "/file/name" or "type:table" patterns,
       separated by commas and/or whitespace. The list is matched left to
       right, and the search stops on the first match. A "/file/name" pattern
       is replaced by its contents; a "type:table" lookup table is matched
       when a name matches a lookup key (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude a user name from the list. The form "!/file/name"
       is supported only in Postfix version 2.4 and later.

       Example:

       authorized_submit_users = !www, static:all

       This feature is available in Postfix 2.2 and later.

authorized_verp_clients (default: $mynetworks)
       What remote SMTP clients are allowed to specify the XVERP command.
       This command requests that mail be delivered one recipient at a time
       with a per recipient return address.

       By default, only trusted clients are allowed to specify XVERP.

       This parameter was introduced with Postfix version 1.1.  Postfix
       version 2.1 renamed this parameter to smtpd_authorized_verp_clients and
       changed the default to none.

       Specify a list of network/netmask patterns, separated by commas and/or
       whitespace. The mask specifies the number of bits in the network part
       of a host address. You can also specify hostnames or .domain names (the
       initial dot causes the domain to match any name below it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is
       replaced by its contents; a "type:table" lookup table is matched when a
       table entry matches a lookup string (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude an address or network block from the list. The
       form "!/file/name" is supported only in Postfix version 2.4 and later.

       Note: IP version 6 address information must be specified inside [] in
       the authorized_verp_clients value, and in files specified with
       "/file/name".  IP version 6 addresses contain the ":" character, and
       would otherwise be confused with a "type:table" pattern.

backwards_bounce_logfile_compatibility (default: yes)
       Produce additional bounce(8) logfile records that can be read by
       Postfix versions before 2.0. The current and more extensible "name =
       value" format is needed in order to implement more sophisticated
       functionality.

       This feature is available in Postfix 2.1 and later.

berkeley_db_create_buffer_size (default: 16777216)
       The per-table I/O buffer size for programs that create Berkeley DB hash
       or btree tables.  Specify a byte count.

       This feature is available in Postfix 2.0 and later.

berkeley_db_read_buffer_size (default: 131072)
       The per-table I/O buffer size for programs that read Berkeley DB hash
       or btree tables.  Specify a byte count.

       This feature is available in Postfix 2.0 and later.

best_mx_transport (default: empty)
       Where the Postfix SMTP client should deliver mail when it detects a
       "mail loops back to myself" error condition. This happens when the
       local MTA is the best SMTP mail exchanger for a destination not listed
       in $mydestination, $inet_interfaces, $proxy_interfaces,
       $virtual_alias_domains, or $virtual_mailbox_domains.  By default, the
       Postfix SMTP client returns such mail as undeliverable.

       Specify, for example, "best_mx_transport = local" to pass the mail from
       the Postfix SMTP client to the local(8) delivery agent. You can specify
       any message delivery "transport" or "transport:nexthop" that is defined
       in the master.cf file. See the transport(5) manual page for the syntax
       and meaning of "transport" or "transport:nexthop".

       However, this feature is expensive because it ties up a Postfix SMTP
       client process while the local(8) delivery agent is doing its work. It
       is more efficient (for Postfix) to list all hosted domains in a table
       or database.

biff (default: yes)
       Whether or not to use the local biff service.  This service sends "new
       mail" notifications to users who have requested new mail notification
       with the UNIX command "biff y".

       For compatibility reasons this feature is on by default.  On systems
       with lots of interactive users, the biff service can be a performance
       drain.  Specify "biff = no" in main.cf to disable.

body_checks (default: empty)
       Optional lookup tables for content inspection as specified in the
       body_checks(5) manual page.

       Note: with Postfix versions before 2.0, these rules inspect all content
       after the primary message headers.

body_checks_size_limit (default: 51200)
       How much text in a message body segment (or attachment, if you prefer
       to use that term) is subjected to body_checks inspection.  The amount
       of text is limited to avoid scanning huge attachments.

       This feature is available in Postfix 2.0 and later.

bounce_notice_recipient (default: postmaster)
       The recipient of postmaster notifications with the message headers of
       mail that Postfix did not deliver and of SMTP conversation transcripts
       of mail that Postfix did not receive.  This feature is enabled with the
       notify_classes parameter.

bounce_queue_lifetime (default: 5d)
       Consider a bounce message as undeliverable, when delivery fails with a
       temporary error, and the time in the queue has reached the
       bounce_queue_lifetime limit.  By default, this limit is the same as for
       regular mail.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       Specify 0 when mail delivery should be tried only once.

       This feature is available in Postfix 2.1 and later.

bounce_service_name (default: bounce)
       The name of the bounce(8) service. This service maintains a record of
       failed delivery attempts and generates non-delivery notifications.

       This feature is available in Postfix 2.0 and later.

bounce_size_limit (default: 50000)
       The maximal amount of original message text that is sent in a
       non-delivery notification. Specify a byte count.  A message is returned
       as either message/rfc822 (the complete original) or as
       text/rfc822-headers (the headers only).  With Postfix version 2.4 and
       earlier, a message is always returned as message/rfc822 and is
       truncated when it exceeds the size limit.

       Notes:

       ⊕      If you increase this limit, then you should increase the
              mime_nesting_limit value proportionally.

       ⊕      Be careful when making changes.  Excessively large values will
              result in the loss of non-delivery notifications, when a bounce
              message size exceeds a local or remote MTA's message size limit.

bounce_template_file (default: empty)
       Pathname of a configuration file with bounce message templates.  These
       override the built-in templates of delivery status notification (DSN)
       messages for undeliverable mail, delayed mail, successful delivery, or
       delivery verification. The bounce(5) manual page describes how to edit
       and test template files.

       Template message body text may contain $name references to Postfix
       configuration parameters. The result of $name expansion can be
       previewed with "postconf -b file_name" before the file is placed into
       the Postfix configuration directory.

       This feature is available in Postfix 2.3 and later.

broken_sasl_auth_clients (default: no)
       Enable interoperability with remote SMTP clients that implement an
       obsolete version of the AUTH command (RFC 4954). Examples of such
       clients are MicroSoft Outlook Express version 4 and MicroSoft Exchange
       version 5.0.

       Specify "broken_sasl_auth_clients = yes" to have Postfix advertise AUTH
       support in a non-standard way.

canonical_classes (default: envelope_sender, envelope_recipient,
       header_sender, header_recipient)
       What addresses are subject to canonical_maps address mapping.  By
       default, canonical_maps address mapping is applied to envelope sender
       and recipient addresses, and to header sender and header recipient
       addresses.

       Specify one or more of: envelope_sender, envelope_recipient,
       header_sender, header_recipient

       This feature is available in Postfix 2.2 and later.

canonical_maps (default: empty)
       Optional address mapping lookup tables for message headers and
       envelopes. The mapping is applied to both sender and recipient
       addresses, in both envelopes and in headers, as controlled with the
       canonical_classes parameter. This is typically used to clean up dirty
       addresses from legacy mail systems, or to replace login names by
       Firstname.Lastname.  The table format and lookups are documented in
       canonical(5). For an overview of Postfix address manipulations see the
       ADDRESS_REWRITING_README document.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.  Note: these lookups are recursive.

       If you use this feature, run "postmap /etc/postfix/canonical" to build
       the necessary DBM or DB file after every change. The changes will
       become visible after a minute or so.  Use "postfix reload" to eliminate
       the delay.

       Note: with Postfix version 2.2, message header address mapping happens
       only when message header address rewriting is enabled:

       ⊕      The message is received with the Postfix sendmail(1) command,

       ⊕      The message is received from a network client that matches
              $local_header_rewrite_clients,

       ⊕      The message is received from the network, and the
              remote_header_rewrite_domain parameter specifies a non-empty
              value.

       To get the behavior before Postfix version 2.2, specify
       "local_header_rewrite_clients = static:all".

       Examples:

       canonical_maps = dbm:/etc/postfix/canonical
       canonical_maps = hash:/etc/postfix/canonical

cleanup_service_name (default: cleanup)
       The name of the cleanup(8) service. This service rewrites addresses
       into the standard form, and performs canonical(5) address mapping and
       virtual(5) aliasing.

       This feature is available in Postfix 2.0 and later.

command_directory (default: see postconf -d output)
       The location of all postfix administrative commands.

command_execution_directory (default: empty)
       The local(8) delivery agent working directory for delivery to external
       commands.  Failure to change directory causes the delivery to be
       deferred.

       The command_execution_directory value is not subject to Postfix
       configuration parameter $name expansion. Instead, the following $name
       expansions are done on command_execution_directory before the directory
       is used. Expansion happens in the context of the delivery request.  The
       result of $name expansion is filtered with the character set that is
       specified with the execution_directory_expansion_filter parameter.

       $user  The recipient's username.

       $shell The recipient's login shell pathname.

       $home  The recipient's home directory.

       $recipient
              The full recipient address.

       $extension
              The optional recipient address extension.

       $domain
              The recipient domain.

       $local The entire recipient localpart.

       $recipient_delimiter
              The address extension delimiter that was found in the recipient
              address (Postfix 2.11 and later), or the system-wide recipient
              address extension delimiter (Postfix 2.10 and earlier).

       ${name?value}

       ${name?{value}} (Postfix >= 3.0)
              Expands to value when $name is non-empty.

       ${name:value}

       ${name:{value}} (Postfix >= 3.0)
              Expands to value when $name is empty.

       ${name?{value1}:{value2}} (Postfix >= 3.0)
              Expands to value1 when $name is non-empty, value2 otherwise.

       Instead of $name you can also specify ${name} or $(name).

       This feature is available in Postfix 2.2 and later.

command_expansion_filter (default: see postconf -d output)
       Restrict the characters that the local(8) delivery agent allows in
       $name expansions of $mailbox_command and $command_execution_directory.
       Characters outside the allowed set are replaced by underscores.

command_time_limit (default: 1000s)
       Time limit for delivery to external commands. This limit is used by the
       local(8) delivery agent, and is the default time limit for delivery by
       the pipe(8) delivery agent.

       Note: if you set this time limit to a large value you must update the
       global ipc_timeout parameter as well.

compatibility_level (default: 0)
       A safety net that causes Postfix to run with backwards-compatible
       default settings after an upgrade to a newer Postfix version.

       With backwards compatibility turned on (the main.cf compatibility_level
       value is less than the Postfix built-in value), Postfix looks for
       settings that are left at their implicit default value, and logs a
       message when a backwards-compatible default setting is required.

           using backwards-compatible default setting name=value
               to [accept a specific client request]

           using backwards-compatible default setting name=value
               to [enable specific Postfix behavior]

       See COMPATIBILITY_README for specific message details. If such a
       message is logged in the context of a legitimate request, the system
       administrator should make the backwards-compatible setting permanent in
       main.cf or master.cf, for example:

           # postconf name=value
           # postfix reload

       When no more backwards-compatible settings need to be made permanent,
       the administrator should turn off backwards compatibility by updating
       the compatibility_level setting in main.cf:

           # postconf compatibility_level=N
           # postfix reload

       For N specify the number that is logged in your postfix(1) warning
       message:

           warning: To disable backwards compatibility use "postconf
               compatibility_level=N" and "postfix reload"

       Starting with Postfix version 3.6, the compatibility level in the above
       warning message is the Postfix version that introduced the last
       incompatible change. The level is formatted as major.minor.patch, where
       patch is usually omitted and defaults to zero. Earlier compatibility
       levels are 0, 1 and 2.

       NOTE: this also introduces support for the "<level", "<=level", and
       other operators to compare compatibility levels.  With the standard
       operators "<", "<=", etc., compatibility level "3.10" would be smaller
       than "3.9" which is undesirable.

       This feature is available in Postfix 3.0 and later.

config_directory (default: see postconf -d output)
       The default location of the Postfix main.cf and master.cf configuration
       files. This can be overruled via the following mechanisms:

       ⊕      The MAIL_CONFIG environment variable (daemon processes and
              commands).

       ⊕      The "-c" command-line option (commands only).

       With Postfix commands that run with set-gid privileges, a
       config_directory override either requires root privileges, or it
       requires that the directory is listed with the
       alternate_config_directories parameter in the default main.cf file.

confirm_delay_cleared (default: no)
       After sending a "your message is delayed" notification, inform the
       sender when the delay clears up. This can result in a sudden burst of
       notifications at the end of a prolonged network outage, and is
       therefore disabled by default.

       See also: delay_warning_time.

       This feature is available in Postfix 3.0 and later.

connection_cache_protocol_timeout (default: 5s)
       Time limit for connection cache connect, send or receive operations.
       The time limit is enforced in the client.

       This feature is available in Postfix 2.3 and later.

connection_cache_service_name (default: scache)
       The name of the scache(8) connection cache service.  This service
       maintains a limited pool of cached sessions.

       This feature is available in Postfix 2.2 and later.

connection_cache_status_update_time (default: 600s)
       How frequently the scache(8) server logs usage statistics with
       connection cache hit and miss rates for logical destinations and for
       physical endpoints.

connection_cache_ttl_limit (default: 2s)
       The maximal time-to-live value that the scache(8) connection cache
       server allows. Requests that specify a larger TTL will be stored with
       the maximum allowed TTL. The purpose of this additional control is to
       protect the infrastructure against careless people. The cache TTL is
       already bounded by $max_idle.

content_filter (default: empty)
       After the message is queued, send the entire message to the specified
       transport:destination. The transport name specifies the first field of
       a mail delivery agent definition in master.cf; the syntax of the
       next-hop destination is described in the manual page of the
       corresponding delivery agent.  More information about external content
       filters is in the Postfix FILTER_README file.

       Notes:

       ⊕      This setting has lower precedence than a FILTER action that is
              specified in an access(5), header_checks(5) or body_checks(5)
              table.

       ⊕      The meaning of an empty next-hop filter destination is version
              dependent.  Postfix 2.7 and later will use the recipient domain;
              earlier versions will use $myhostname.  Specify
              "default_filter_nexthop = $myhostname" for compatibility with
              Postfix 2.6 or earlier, or specify a content_filter value with
              an explicit next-hop destination.

cyrus_sasl_config_path (default: empty)
       Search path for Cyrus SASL application configuration files, currently
       used only to locate the $smtpd_sasl_path.conf file.  Specify zero or
       more directories separated by a colon character, or an empty value to
       use Cyrus SASL's built-in search path.

       This feature is available in Postfix 2.5 and later when compiled with
       Cyrus SASL 2.1.22 or later.

daemon_directory (default: see postconf -d output)
       The directory with Postfix support programs and daemon programs.  These
       should not be invoked directly by humans. The directory must be owned
       by root.

daemon_table_open_error_is_fatal (default: no)
       How a Postfix daemon process handles errors while opening lookup
       tables: gradual degradation or immediate termination.

        no  (default)
              Gradual degradation: a daemon process logs a message of type
              "error" and continues execution with reduced functionality.
              Features that do not depend on the unavailable table will work
              normally, while features that depend on the table will result in
              a type "warning" message.
              When the notify_classes parameter value contains the "data"
              class, the Postfix SMTP server and client will report
              transcripts of sessions with an error because a table is
              unavailable.

        yes  (historical behavior)
              Immediate termination: a daemon process logs a type "fatal"
              message and terminates immediately.  This option reduces the
              number of possible code paths through Postfix, and may therefore
              be slightly more secure than the default.

       For the sake of sanity, the number of type "error" messages is limited
       to 13 over the lifetime of a daemon process.

       This feature is available in Postfix 2.9 and later.

daemon_timeout (default: 18000s)
       How much time a Postfix daemon process may take to handle a request
       before it is terminated by a built-in watchdog timer.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

data_directory (default: see postconf -d output)
       The directory with Postfix-writable data files (for example: caches,
       pseudo-random numbers).  This directory must be owned by the mail_owner
       account, and must not be shared with non-Postfix software.

       This feature is available in Postfix 2.5 and later.

debug_peer_level (default: 2)
       The increment in verbose logging level when a nexthop destination,
       remote client or server name or network address matches a pattern given
       with the debug_peer_list parameter.

       Per-nexthop debug logging is available in Postfix 3.6 and later.

debug_peer_list (default: empty)
       Optional list of nexthop destination, remote client or server name or
       network address patterns that, if matched, cause the verbose logging
       level to increase by the amount specified in $debug_peer_level.

       Per-nexthop debug logging is available in Postfix 3.6 and later.

       Specify domain names, network/netmask patterns, "/file/name" patterns
       or "type:table" lookup tables. The right-hand side result from
       "type:table" lookups is ignored.

       Pattern matching of domain names is controlled by the presence or
       absence of "debug_peer_list" in the parent_domain_matches_subdomains
       parameter value.

       Examples:

       debug_peer_list = 127.0.0.1
       debug_peer_list = example.com

debugger_command (default: empty)
       The external command to execute when a Postfix daemon program is
       invoked with the -D option.

       Use "command .. & sleep 5" so that the debugger can attach before the
       process marches on. If you use an X-based debugger, be sure to set up
       your XAUTHORITY environment variable before starting Postfix.

       Note: the command is subject to $name expansion, before it is passed to
       the default command interpreter. Specify "$$" to produce a single "$"
       character.

       Example:

       debugger_command =
           PATH=/usr/bin:/usr/X11R6/bin
           ddd $daemon_directory/$process_name $process_id & sleep 5

default_database_type (default: see postconf -d output)
       The default database type for use in newaliases(1), postalias(1) and
       postmap(1) commands. On many UNIX systems the default type is either
       dbm or hash. The default setting is frozen when the Postfix system is
       built.

       Examples:

       default_database_type = hash
       default_database_type = dbm

default_delivery_slot_cost (default: 5)
       How often the Postfix queue manager's scheduler is allowed to preempt
       delivery of one message with another.

       Each transport maintains a so-called "available delivery slot counter"
       for each message. One message can be preempted by another one when the
       other message can be delivered using no more delivery slots (i.e.,
       invocations of delivery agents) than the current message counter has
       accumulated (or will eventually accumulate - see about slot loans
       below). This parameter controls how often the counter is incremented -
       it happens after each default_delivery_slot_cost recipients have been
       delivered.

       The cost of 0 is used to disable the preempting scheduling completely.
       The minimum value the scheduling algorithm can use is 2 - use it if you
       want to maximize the message throughput rate. Although there is no
       maximum, it doesn't make much sense to use values above say 50.

       The only reason why the value of 2 is not the default is the way this
       parameter affects the delivery of mailing-list mail. In the worst case,
       delivery can take somewhere between (cost+1/cost) and (cost/cost-1)
       times more than if the preemptive scheduler was disabled. The default
       value of 5 turns out to provide reasonable message response times while
       making sure the mailing-list deliveries are not extended by more than
       20-25 percent even in the worst case.

       Use transport_delivery_slot_cost to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

       Examples:

       default_delivery_slot_cost = 0
       default_delivery_slot_cost = 2

default_delivery_slot_discount (default: 50)
       The default value for transport-specific _delivery_slot_discount
       settings.

       This parameter speeds up the moment when a message preemption can
       happen. Instead of waiting until the full amount of delivery slots
       required is available, the preemption can happen when
       transport_delivery_slot_discount percent of the required amount plus
       transport_delivery_slot_loan still remains to be accumulated.  Note
       that the full amount will still have to be accumulated before another
       preemption can take place later.

       Use transport_delivery_slot_discount to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

default_delivery_slot_loan (default: 3)
       The default value for transport-specific _delivery_slot_loan settings.

       This parameter speeds up the moment when a message preemption can
       happen. Instead of waiting until the full amount of delivery slots
       required is available, the preemption can happen when
       transport_delivery_slot_discount percent of the required amount plus
       transport_delivery_slot_loan still remains to be accumulated.  Note
       that the full amount will still have to be accumulated before another
       preemption can take place later.

       Use transport_delivery_slot_loan to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

default_delivery_status_filter (default: empty)
       Optional filter to replace the delivery status code or explanatory text
       of successful or unsuccessful deliveries.  This does not allow the
       replacement of a successful status code (2.X.X) with an unsuccessful
       status code (4.X.X or 5.X.X) or vice versa.

       Note: the (smtp|lmtp)_delivery_status_filter is applied only once per
       recipient: when delivery is successful, when delivery is rejected with
       5XX, or when there are no more alternate MX or A destinations. Use
       smtp_reply_filter or lmtp_reply_filter to inspect responses for all
       delivery attempts.

       The following parameters can be used to implement a filter for specific
       delivery agents: lmtp_delivery_status_filter,
       local_delivery_status_filter, pipe_delivery_status_filter,
       smtp_delivery_status_filter or virtual_delivery_status_filter. These
       parameters support the same filter syntax as described here.

       Specify zero or more "type:table" lookup table names, separated by
       comma or whitespace. For each successful or unsuccessful delivery to a
       recipient, the tables are queried in the specified order with one line
       of text that is structured as follows:

           enhanced-status-code SPACE explanatory-text

       The first table match wins. The lookup result must have the same
       structure as the query, a successful status code (2.X.X) must be
       replaced with a successful status code, an unsuccessful status code
       (4.X.X or 5.X.X) must be replaced with an unsuccessful status code, and
       the explanatory text field must be non-empty. Other results will result
       in a warning.

       Example 1: convert specific soft TLS errors into hard errors, by
       overriding the first number in the enhanced status code.

           /etc/postfix/main.cf:
               smtp_delivery_status_filter = pcre:/etc/postfix/smtp_dsn_filter

           /etc/postfix/smtp_dsn_filter:
               /^4(\.\d+\.\d+ TLS is required, but host \S+ refused to start TLS: .+)/
                   5$1
               /^4(\.\d+\.\d+ TLS is required, but was not offered by host .+)/
                   5$1
               # Do not change the following into hard bounces. They may
               # result from a local configuration problem.
               # 4.\d+.\d+ TLS is required, but our TLS engine is unavailable
               # 4.\d+.\d+ TLS is required, but unavailable
               # 4.\d+.\d+ Cannot start TLS: handshake failure

       Example 2: censor the per-recipient delivery status text so that it
       does not reveal the destination command or filename when a remote
       sender requests confirmation of successful delivery.

           /etc/postfix/main.cf:
               local_delivery_status_filter = pcre:/etc/postfix/local_dsn_filter

           /etc/postfix/local_dsn_filter:
               /^(2\S+ delivered to file).+/    $1
               /^(2\S+ delivered to command).+/ $1

       Notes:

       ⊕      This feature will NOT override the soft_bounce safety net.

       ⊕      This feature will change the enhanced status code and text that
              is logged to the maillog file, and that is reported to the
              sender in delivery confirmation or non-delivery notifications.

       This feature is available in Postfix 3.0 and later.

default_destination_concurrency_failed_cohort_limit (default: 1)
       How many pseudo-cohorts must suffer connection or handshake failure
       before a specific destination is considered unavailable (and further
       delivery is suspended). Specify zero to disable this feature. A
       destination's pseudo-cohort failure count is reset each time a delivery
       completes without connection or handshake failure for that specific
       destination.

       A pseudo-cohort is the number of deliveries equal to a destination's
       delivery concurrency.

       Use transport_destination_concurrency_failed_cohort_limit to specify a
       transport-specific override, where transport is the master.cf name of
       the message delivery transport.

       This feature is available in Postfix 2.5. The default setting is
       compatible with earlier Postfix versions.

default_destination_concurrency_limit (default: 20)
       The default maximal number of parallel deliveries to the same
       destination.  This is the default limit for delivery via the lmtp(8),
       pipe(8), smtp(8) and virtual(8) delivery agents.  With a
       per-destination recipient limit > 1, a destination is a domain,
       otherwise it is a recipient.

       Use transport_destination_concurrency_limit to specify a
       transport-specific override, where transport is the master.cf name of
       the message delivery transport.

default_destination_concurrency_negative_feedback (default: 1)
       The per-destination amount of delivery concurrency negative feedback,
       after a delivery completes with a connection or handshake failure.
       Feedback values are in the range 0..1 inclusive. With negative
       feedback, concurrency is decremented at the beginning of a sequence of
       length 1/feedback. This is unlike positive feedback, where concurrency
       is incremented at the end of a sequence of length 1/feedback.

       As of Postfix version 2.5, negative feedback cannot reduce delivery
       concurrency to zero.  Instead, a destination is marked dead (further
       delivery suspended) after the failed pseudo-cohort count reaches
       $default_destination_concurrency_failed_cohort_limit (or
       $transport_destination_concurrency_failed_cohort_limit).  To make the
       scheduler completely immune to connection or handshake failures,
       specify a zero feedback value and a zero failed pseudo-cohort limit.

       Specify one of the following forms:

       number

       number / number
              Constant feedback. The value must be in the range 0..1
              inclusive.  The default setting of "1" is compatible with
              Postfix versions before 2.5, where a destination's delivery
              concurrency is throttled down to zero (and further delivery
              suspended) after a single failed pseudo-cohort.

       number / concurrency
              Variable feedback of "number / (delivery concurrency)".  The
              number must be in the range 0..1 inclusive. With number equal to
              "1", a destination's delivery concurrency is decremented by 1
              after each failed pseudo-cohort.

       A pseudo-cohort is the number of deliveries equal to a destination's
       delivery concurrency.

       Use transport_destination_concurrency_negative_feedback to specify a
       transport-specific override, where transport is the master.cf name of
       the message delivery transport.

       This feature is available in Postfix 2.5. The default setting is
       compatible with earlier Postfix versions.

default_destination_concurrency_positive_feedback (default: 1)
       The per-destination amount of delivery concurrency positive feedback,
       after a delivery completes without connection or handshake failure.
       Feedback values are in the range 0..1 inclusive.  The concurrency
       increases until it reaches the per-destination maximal concurrency
       limit. With positive feedback, concurrency is incremented at the end of
       a sequence with length 1/feedback. This is unlike negative feedback,
       where concurrency is decremented at the start of a sequence of length
       1/feedback.

       Specify one of the following forms:

       number

       number / number
              Constant feedback.  The value must be in the range 0..1
              inclusive. The default setting of "1" is compatible with Postfix
              versions before 2.5, where a destination's delivery concurrency
              doubles after each successful pseudo-cohort.

       number / concurrency
              Variable feedback of "number / (delivery concurrency)".  The
              number must be in the range 0..1 inclusive. With number equal to
              "1", a destination's delivery concurrency is incremented by 1
              after each successful pseudo-cohort.

       A pseudo-cohort is the number of deliveries equal to a destination's
       delivery concurrency.

       Use transport_destination_concurrency_positive_feedback to specify a
       transport-specific override, where transport is the master.cf name of
       the message delivery transport.

       This feature is available in Postfix 2.5 and later.

default_destination_rate_delay (default: 0s)
       The default amount of delay that is inserted between individual message
       deliveries to the same destination and over the same message delivery
       transport. Specify a non-zero value to rate-limit those message
       deliveries to at most one per $default_destination_rate_delay.

       The resulting behavior depends on the value of the corresponding
       per-destination recipient limit.

       ⊕      With a corresponding per-destination recipient limit > 1, the
              rate delay specifies the time between deliveries to the same
              domain.  Different domains are delivered in parallel, subject to
              the process limits specified in master.cf.

       ⊕      With a corresponding per-destination recipient limit equal to 1,
              the rate delay specifies the time between deliveries to the same
              recipient. Different recipients are delivered in parallel,
              subject to the process limits specified in master.cf.

       To enable the delay, specify a non-zero time value (an integral value
       plus an optional one-letter suffix that specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
       The default time unit is s (seconds).

       NOTE: the delay is enforced by the queue manager. The delay timer state
       does not survive "postfix reload" or "postfix stop".

       Use transport_destination_rate_delay to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

       NOTE: with a non-zero _destination_rate_delay, specify a
       transport_destination_concurrency_failed_cohort_limit of 10 or more to
       prevent Postfix from deferring all mail for the same destination after
       only one connection or handshake error.

       This feature is available in Postfix 2.5 and later.

default_destination_recipient_limit (default: 50)
       The default maximal number of recipients per message delivery.  This is
       the default limit for delivery via the lmtp(8), pipe(8), smtp(8) and
       virtual(8) delivery agents.

       Setting this parameter to a value of 1 affects email deliveries as
       follows:

       ⊕      It changes the meaning of the corresponding per-destination
              concurrency limit, from concurrency of deliveries to the same
              domain into concurrency of deliveries to the same recipient.
              Different recipients are delivered in parallel, subject to the
              process limits specified in master.cf.

       ⊕      It changes the meaning of the corresponding per-destination rate
              delay, from the delay between deliveries to the same domain into
              the delay between deliveries to the same recipient.  Again,
              different recipients are delivered in parallel, subject to the
              process limits specified in master.cf.

       ⊕      It changes the meaning of other corresponding per-destination
              settings in a similar manner, from settings for delivery to the
              same domain into settings for delivery to the same recipient.

       Use transport_destination_recipient_limit to specify a
       transport-specific override, where transport is the master.cf name of
       the message delivery transport.

default_extra_recipient_limit (default: 1000)
       The default value for the extra per-transport limit imposed on the
       number of in-memory recipients.  This extra recipient space is reserved
       for the cases when the Postfix queue manager's scheduler preempts one
       message with another and suddenly needs some extra recipient slots for
       the chosen message in order to avoid performance degradation.

       Use transport_extra_recipient_limit to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

default_filter_nexthop (default: empty)
       When a content_filter or FILTER request specifies no explicit next-hop
       destination, use $default_filter_nexthop instead; when that value is
       empty, use the domain in the recipient address.  Specify
       "default_filter_nexthop = $myhostname" for compatibility with Postfix
       version 2.6 and earlier, or specify an explicit next-hop destination
       with each content_filter value or FILTER action.

       This feature is available in Postfix 2.7 and later.

default_minimum_delivery_slots (default: 3)
       How many recipients a message must have in order to invoke the Postfix
       queue manager's scheduling algorithm at all.  Messages which would
       never accumulate at least this many delivery slots (subject to slot
       cost parameter as well) are never preempted.

       Use transport_minimum_delivery_slots to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

default_privs (default: nobody)
       The default rights used by the local(8) delivery agent for delivery to
       an external file or command.  These rights are used when delivery is
       requested from an aliases(5) file that is owned by root, or when
       delivery is done on behalf of root. DO NOT SPECIFY A PRIVILEGED USER OR
       THE POSTFIX OWNER.

default_process_limit (default: 100)
       The default maximal number of Postfix child processes that provide a
       given service. This limit can be overruled for specific services in the
       master.cf file.

default_rbl_reply (default: see postconf -d output)
       The default Postfix SMTP server response template for a request that is
       rejected by an RBL-based restriction. This template can be overruled by
       specific entries in the optional rbl_reply_maps lookup table.

       This feature is available in Postfix 2.0 and later.

       The template does not support Postfix configuration parameter $name
       substitution. Instead, it supports exactly one level of $name
       substitution for the following attributes:

       $client
              The client hostname and IP address, formatted as name[address].

       $client_address
              The client IP address.

       $client_name
              The client hostname or "unknown". See
              reject_unknown_client_hostname for more details.

       $reverse_client_name
              The client hostname from address->name lookup, or "unknown".
              See reject_unknown_reverse_client_hostname for more details.

       $helo_name
              The hostname given in HELO or EHLO command or empty string.

       $rbl_class
              The denylisted entity type: Client host, Helo command, Sender
              address, or Recipient address.

       $rbl_code
              The numerical SMTP response code, as specified with the
              maps_rbl_reject_code configuration parameter. Note: The
              numerical SMTP response code is required, and must appear at the
              start of the reply. With Postfix version 2.3 and later this
              information may be followed by an RFC 3463 enhanced status code.

       $rbl_domain
              The RBL domain where $rbl_what is denylisted.

       $rbl_reason
              The reason why $rbl_what is denylisted, or an empty string.

       $rbl_what
              The entity that is denylisted (an IP address, a hostname, a
              domain name, or an email address whose domain was denylisted).

       $recipient
              The recipient address or <> in case of the null address.

       $recipient_domain
              The recipient domain or empty string.

       $recipient_name
              The recipient address localpart or <> in case of null address.

       $sender
              The sender address or <> in case of the null address.

       $sender_domain
              The sender domain or empty string.

       $sender_name
              The sender address localpart or <> in case of the null address.

       ${name?value}

       ${name?{value}} (Postfix >= 3.0)
              Expands to value when $name is non-empty.

       ${name:value}

       ${name:{value}} (Postfix >= 3.0)
              Expands to value when $name is empty.

       ${name?{value1}:{value2}} (Postfix >= 3.0)
              Expands to value1 when $name is non-empty, value2 otherwise.

       Instead of $name you can also specify ${name} or $(name).

       Note: when an enhanced status code is specified in an RBL reply
       template, it is subject to modification.  The following transformations
       are needed when the same RBL reply template is used for client, helo,
       sender, or recipient access restrictions.

       ⊕      When rejecting a sender address, the Postfix SMTP server will
              transform a recipient DSN status (e.g., 4.1.1-4.1.6) into the
              corresponding sender DSN status, and vice versa.

       ⊕      When rejecting non-address information (such as the HELO command
              argument or the client hostname/address), the Postfix SMTP
              server will transform a sender or recipient DSN status into a
              generic non-address DSN status (e.g., 4.0.0).

default_recipient_limit (default: 20000)
       The default per-transport upper limit on the number of in-memory
       recipients.  These limits take priority over the global
       qmgr_message_recipient_limit after the message has been assigned to the
       respective transports.  See also default_extra_recipient_limit and
       qmgr_message_recipient_minimum.

       Use transport_recipient_limit to specify a transport-specific override,
       where transport is the master.cf name of the message delivery
       transport.

default_recipient_refill_delay (default: 5s)
       The default per-transport maximum delay between refilling recipients.
       When not all message recipients fit into memory at once, keep loading
       more of them at least once every this many seconds.  This is used to
       make sure the recipients are refilled in a timely manner even when
       $default_recipient_refill_limit is too high for too slow deliveries.

       Use transport_recipient_refill_delay to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

       This feature is available in Postfix 2.4 and later.

default_recipient_refill_limit (default: 100)
       The default per-transport limit on the number of recipients refilled at
       once.  When not all message recipients fit into memory at once, keep
       loading more of them in batches of at least this many at a time.  See
       also $default_recipient_refill_delay, which may result in recipient
       batches lower than this when this limit is too high for too slow
       deliveries.

       Use transport_recipient_refill_limit to specify a transport-specific
       override, where transport is the master.cf name of the message delivery
       transport.

       This feature is available in Postfix 2.4 and later.

default_transport (default: smtp)
       The default mail delivery transport and next-hop destination for
       destinations that do not match $mydestination, $inet_interfaces,
       $proxy_interfaces, $virtual_alias_domains, $virtual_mailbox_domains, or
       $relay_domains.  This information can be overruled with the
       sender_dependent_default_transport_maps parameter and with the
       transport(5) table.

       In order of decreasing precedence, the nexthop destination is taken
       from $sender_dependent_default_transport_maps, $default_transport,
       $sender_dependent_relayhost_maps, $relayhost, or from the recipient
       domain.

       Specify a string of the form transport:nexthop, where transport is the
       name of a mail delivery transport defined in master.cf.  The :nexthop
       destination is optional; its syntax is documented in the manual page of
       the corresponding delivery agent. In the case of SMTP or LMTP, specify
       one or more destinations separated by comma or whitespace (with Postfix
       3.5 and later).

       Example:

       default_transport = uucp:relayhostname

default_transport_rate_delay (default: 0s)
       The default amount of delay that is inserted between individual message
       deliveries over the same message delivery transport, regardless of
       destination. Specify a non-zero value to rate-limit those message
       deliveries to at most one per $default_transport_rate_delay.

       Use transport_transport_rate_delay to specify a transport-specific
       override, where the initial transport is the master.cf name of the
       message delivery transport.

       Example: throttle outbound SMTP mail to at most 3 deliveries per
       minute.

       /etc/postfix/main.cf:
           smtp_transport_rate_delay = 20s

       To enable the delay, specify a non-zero time value (an integral value
       plus an optional one-letter suffix that specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
       The default time unit is s (seconds).

       NOTE: the delay is enforced by the queue manager.

       This feature is available in Postfix 3.1 and later.

default_verp_delimiters (default: +=)
       The two default VERP delimiter characters. These are used when no
       explicit delimiters are specified with the SMTP XVERP command or with
       the "sendmail -XV" command-line option (Postfix 2.2 and earlier: -V).
       Specify characters that are allowed by the verp_delimiter_filter
       setting.

       This feature is available in Postfix 1.1 and later.

defer_code (default: 450)
       The numerical Postfix SMTP server response code when a remote SMTP
       client request is rejected by the "defer" restriction.

       Do not change this unless you have a complete understanding of RFC
       5321.

defer_service_name (default: defer)
       The name of the defer service. This service is implemented by the
       bounce(8) daemon and maintains a record of failed delivery attempts and
       generates non-delivery notifications.

       This feature is available in Postfix 2.0 and later.

defer_transports (default: empty)
       The names of message delivery transports that should not deliver mail
       unless someone issues "sendmail -q" or equivalent. Specify zero or more
       mail delivery transport names that appear in the first field of
       master.cf.

       Example:

       defer_transports = smtp

delay_logging_resolution_limit (default: 2)
       The maximal number of digits after the decimal point when logging
       sub-second delay values.  Specify a number in the range 0..6.

       Large delay values are rounded off to an integral number of seconds;
       delay values below the delay_logging_resolution_limit are logged as
       "0", and delay values under 100s are logged with at most two-digit
       precision.

       The format of the "delays=a/b/c/d" logging is as follows:

       ⊕      a = time from message arrival to last active queue entry

       ⊕      b = time from last active queue entry to connection setup

       ⊕      c = time in connection setup, including DNS, EHLO and STARTTLS

       ⊕      d = time in message transmission

       This feature is available in Postfix 2.3 and later.

delay_notice_recipient (default: postmaster)
       The recipient of postmaster notifications with the message headers of
       mail that cannot be delivered within $delay_warning_time time units.

       See also: delay_warning_time, notify_classes.

delay_warning_time (default: 0h)
       The time after which the sender receives a copy of the message headers
       of mail that is still queued. The confirm_delay_cleared parameter
       controls sender notification when the delay clears up.

       To enable this feature, specify a non-zero time value (an integral
       value plus an optional one-letter suffix that specifies the time unit).

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
       The default time unit is h (hours).

       See also: delay_notice_recipient, notify_classes,
       confirm_delay_cleared.

deliver_lock_attempts (default: 20)
       The maximal number of attempts to acquire an exclusive lock on a
       mailbox file or bounce(8) logfile.

deliver_lock_delay (default: 1s)
       The time between attempts to acquire an exclusive lock on a mailbox
       file or bounce(8) logfile.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

destination_concurrency_feedback_debug (default: no)
       Make the queue manager's feedback algorithm verbose for performance
       analysis purposes.

       This feature is available in Postfix 2.5 and later.

detect_8bit_encoding_header (default: yes)
       Automatically detect 8BITMIME body content by looking at
       Content-Transfer-Encoding: message headers; historically, this behavior
       was hard-coded to be "always on".

       This feature is available in Postfix 2.5 and later.

disable_dns_lookups (default: no)
       Disable DNS lookups in the Postfix SMTP and LMTP clients. When
       disabled, hosts are looked up with the getaddrinfo() system library
       routine which normally also looks in /etc/hosts.  As of Postfix 2.11,
       this parameter is deprecated; use smtp_dns_support_level instead.

       DNS lookups are enabled by default.

disable_mime_input_processing (default: no)
       Turn off MIME processing while receiving mail. This means that no
       special treatment is given to Content-Type: message headers, and that
       all text after the initial message headers is considered to be part of
       the message body.

       This feature is available in Postfix 2.0 and later.

       Mime input processing is enabled by default, and is needed in order to
       recognize MIME headers in message content.

disable_mime_output_conversion (default: no)
       Disable the conversion of 8BITMIME format to 7BIT format.  Mime output
       conversion is needed when the destination does not advertise 8BITMIME
       support.

       This feature is available in Postfix 2.0 and later.

disable_verp_bounces (default: no)
       Disable sending one bounce report per recipient.

       The default, one per recipient, is what ezmlm needs.

       This feature is available in Postfix 1.1 and later.

disable_vrfy_command (default: no)
       Disable the SMTP VRFY command. This stops some techniques used to
       harvest email addresses.

       Example:

       disable_vrfy_command = no

dns_ncache_ttl_fix_enable (default: no)
       Enable a workaround for future libc incompatibility. The Postfix
       implementation of RFC 2308 negative reply caching relies on the promise
       that res_query() and res_search() invoke res_send(), which returns the
       server response in an application buffer even if the requested record
       does not exist. If this promise is broken, specify "yes" to enable a
       workaround for DNS reputation lookups.

       This feature is available in Postfix 3.1 and later.

dnsblog_reply_delay (default: 0s)
       A debugging aid to artificially delay DNS responses.

       This feature is available in Postfix 2.8.

dnsblog_service_name (default: dnsblog)
       The name of the dnsblog(8) service entry in master.cf. This service
       performs DNS allow/denylist lookups.

       This feature is available in Postfix 2.8 and later.

dnssec_probe (default: ns:.)
       The DNS query type (default: "ns") and DNS query name (default: ".")
       that Postfix may use to determine whether DNSSEC validation is
       available.

       Background: DNSSEC validation is needed for Postfix DANE support; this
       ensures that Postfix receives TLSA records with secure TLS server
       certificate info. When DNSSEC validation is unavailable, mail
       deliveries using opportunistic DANE will not be protected by server
       certificate info in TLSA records, and mail deliveries using mandatory
       DANE will not be made at all.

       By default, a Postfix process will send a DNSSEC probe after 1) the
       process made a DNS query that requested DNSSEC validation, 2) the
       process did not receive a DNSSEC validated response to this query or to
       an earlier query, and 3) the process did not already send a DNSSEC
       probe.

       When the DNSSEC probe has no response, or when the response is not
       DNSSEC validated, Postfix logs a warning that DNSSEC validation may be
       unavailable.

       Example:

       warning: DNSSEC validation may be unavailable
       warning: reason: dnssec_probe 'ns:.' received a response that is not DNSSEC validated
       warning: reason: dnssec_probe 'ns:.' received no response: Server failure

       Possible reasons why DNSSEC validation may be unavailable:

       ⊕      The local /etc/resolv.conf file specifies a DNS resolver that
              does not validate DNSSEC signatures (that's
              $queue_directory/etc/resolv.conf when a Postfix daemon runs in a
              chroot jail).

       ⊕      The local system library does not pass on the "DNSSEC validated"
              bit to Postfix, or Postfix does not know how to ask the library
              to do that.

       By default, the DNSSEC probe asks for the DNS root zone NS records,
       because resolvers should always have that information cached. If
       Postfix runs on a network where the DNS root zone is not reachable,
       specify a different probe, or specify an empty dnssec_probe value to
       disable the feature.

       This feature is available in Postfix 3.6 and later. It was backported
       to Postfix versions 3.5.9, 3.4.19, 3.3.16. 3.2.21.

dont_remove (default: 0)
       Don't remove queue files and save them to the "saved" mail queue.  This
       is a debugging aid.  To inspect the envelope information and content of
       a Postfix queue file, use the postcat(1) command.

double_bounce_sender (default: double-bounce)
       The sender address of postmaster notifications that are generated by
       the mail system. All mail to this address is silently discarded, in
       order to terminate mail bounce loops.

duplicate_filter_limit (default: 1000)
       The maximal number of addresses remembered by the address duplicate
       filter for aliases(5) or virtual(5) alias expansion, or for showq(8)
       queue displays.

empty_address_default_transport_maps_lookup_key (default: <>)
       The sender_dependent_default_transport_maps search string that will be
       used instead of the null sender address.

       This feature is available in Postfix 2.7 and later.

empty_address_local_login_sender_maps_lookup_key (default: <>)
       The lookup key to be used in local_login_sender_maps tables, instead of
       the null sender address.

       This feature is available in Postfix 3.6 and later.

empty_address_recipient (default: MAILER-DAEMON)
       The recipient of mail addressed to the null address.  Postfix does not
       accept such addresses in SMTP commands, but they may still be created
       locally as the result of configuration or software error.

empty_address_relayhost_maps_lookup_key (default: <>)
       The sender_dependent_relayhost_maps search string that will be used
       instead of the null sender address.

       This feature is available in Postfix 2.5 and later. With earlier
       versions, sender_dependent_relayhost_maps lookups were skipped for the
       null sender address.

enable_errors_to (default: no)
       Report mail delivery errors to the address specified with the
       non-standard Errors-To: message header, instead of the envelope sender
       address (this feature is removed with Postfix version 2.2, is turned
       off by default with Postfix version 2.1, and is always turned on with
       older Postfix versions).

enable_idna2003_compatibility (default: no)
       Enable 'transitional' compatibility between IDNA2003 and IDNA2008, when
       converting UTF-8 domain names to/from the ASCII form that is used for
       DNS lookups. Specify "yes" for compatibility with Postfix <= 3.1 (not
       recommended). This affects the conversion of domain names that contain
       for example the German sz and the Greek zeta.  See
       http://unicode.org/cldr/utility/idna.jsp for more examples.

       This feature is available in Postfix 3.2 and later.

enable_long_queue_ids (default: no)
       Enable long, non-repeating, queue IDs (queue file names).  The benefit
       of non-repeating names is simpler logfile analysis and easier queue
       migration (there is no need to run "postsuper" to change queue file
       names that don't match their message file inode number).

       Note: see below for how to convert long queue file names to Postfix <=
       2.8.

       Changing the parameter value to "yes" has the following effects:

       ⊕      Existing queue file names are not affected.

       ⊕      New queue files are created with names such as 3Pt2mN2VXxznjll.
              These are encoded in a 52-character alphabet that contains
              digits (0-9), upper-case letters (B-Z) and lower-case letters
              (b-z). For safety reasons the vowels (AEIOUaeiou) are excluded
              from the alphabet.  The name format is: 6 or more characters for
              the time in seconds, 4 characters for the time in microseconds,
              the 'z'; the remainder is the file inode number encoded in the
              first 51 characters of the 52-character alphabet.

       ⊕      New messages have a Message-ID header with queueID@myhostname.

       ⊕      The mailq (postqueue -p) output has a wider Queue ID column.
              The number of whitespace-separated fields is not changed.

       ⊕      The hash_queue_depth algorithm uses the first characters of the
              queue file creation time in microseconds, after conversion into
              hexadecimal representation. This produces the same queue hashing
              behavior as if the queue file name was created with
              "enable_long_queue_ids = no".

       Changing the parameter value to "no" has the following effects:

       ⊕      Existing long queue file names are renamed to the short form
              (while running "postfix reload" or "postsuper").

       ⊕      New queue files are created with names such as C3CD21F3E90 from
              a hexadecimal alphabet that contains digits (0-9) and upper-case
              letters (A-F). The name format is: 5 characters for the time in
              microseconds; the remainder is the file inode number.

       ⊕      New messages have a Message-ID header with
              YYYYMMDDHHMMSS.queueid@myhostname, where YYYYMMDDHHMMSS are the
              year, month, day, hour, minute and second.

       ⊕      The mailq (postqueue -p) output has the same format as with
              Postfix <= 2.8.

       ⊕      The hash_queue_depth algorithm uses the first characters of the
              queue file name, with the hexadecimal representation of the file
              creation time in microseconds.

       Before migration to Postfix <= 2.8, the following commands are required
       to convert long queue file names into short names:

       # postfix stop
       # postconf enable_long_queue_ids=no
       # postsuper

       Repeat the postsuper command until it reports no more queue file name
       changes.

       This feature is available in Postfix 2.9 and later.

enable_original_recipient (default: yes)
       Enable support for the original recipient address after an address is
       rewritten to a different address (for example with aliasing or with
       canonical mapping).

       The original recipient address is used as follows:

       Final delivery
              With "enable_original_recipient = yes", the original recipient
              address is stored in the X-Original-To message header. This
              header may be used to distinguish between different recipients
              that share the same mailbox.

       Recipient deduplication
              With "enable_original_recipient = yes", the cleanup(8) daemon
              performs duplicate recipient elimination based on the content of
              (original recipient, maybe-rewritten recipient) pairs.
              Otherwise, the cleanup(8) daemon performs duplicate recipient
              elimination based only on the maybe-rewritten recipient address.

       Note: with Postfix <= 3.2 the "setting enable_original_recipient = no"
       breaks address verification for addresses that are aliased or otherwise
       rewritten (Postfix is unable to store the address verification result
       under the original probe destination address; instead, it can store the
       result only under the rewritten address).

       This feature is available in Postfix 2.1 and later. Postfix version 2.0
       behaves as if this parameter is always set to yes.  Postfix versions
       before 2.0 have no support for the original recipient address.

enable_threaded_bounces (default: no)
       Enable non-delivery, success, and delay notifications that link to the
       original message by including a References: and In-Reply-To: header
       with the original Message-ID value. There are advantages and
       disadvantages to consider.

        advantage
              This allows mail readers to present a delivery status
              notification in the same email thread as the original message.

        disadvantage
              This makes it easy for users to mistakenly delete the whole
              email thread (all related messages), instead of deleting only
              the non-delivery notification.

       This feature is available in Postfix 3.6 and later.

error_notice_recipient (default: postmaster)
       The recipient of postmaster notifications about mail delivery problems
       that are caused by policy, resource, software or protocol errors.
       These notifications are enabled with the notify_classes parameter.

error_service_name (default: error)
       The name of the error(8) pseudo delivery agent. This service always
       returns mail as undeliverable.

       This feature is available in Postfix 2.0 and later.

execution_directory_expansion_filter (default: see postconf -d output)
       Restrict the characters that the local(8) delivery agent allows in
       $name expansions of $command_execution_directory.  Characters outside
       the allowed set are replaced by underscores.

       This feature is available in Postfix 2.2 and later.

expand_owner_alias (default: no)
       When delivering to an alias "aliasname" that has an "owner-aliasname"
       companion alias, set the envelope sender address to the expansion of
       the "owner-aliasname" alias.  Normally, Postfix sets the envelope
       sender address to the name of the "owner-aliasname" alias.

export_environment (default: see postconf -d output)
       The list of environment variables that a Postfix process will export to
       non-Postfix processes. The TZ variable is needed for sane time keeping
       on System-V-ish systems.

       Specify a list of names and/or name=value pairs, separated by
       whitespace or comma. Specify "{ name=value }" to protect whitespace or
       comma in parameter values (whitespace after the opening "{" and before
       the closing "}" is ignored). The form name=value is supported with
       Postfix version 2.1 and later; the use of {} is supported with Postfix
       3.0 and later.

       Example:

       export_environment = TZ PATH=/bin:/usr/bin

extract_recipient_limit (default: 10240)
       The maximal number of recipient addresses that Postfix will extract
       from message headers when mail is submitted with "sendmail -t".

       This feature was removed in Postfix version 2.1.

fallback_relay (default: empty)
       Optional list of relay hosts for SMTP destinations that can't be found
       or that are unreachable. With Postfix 2.3 this parameter is renamed to
       smtp_fallback_relay.

       By default, mail is returned to the sender when a destination is not
       found, and delivery is deferred when a destination is unreachable.

       The fallback relays must be SMTP destinations. Specify a domain, host,
       host:port, [host]:port, [address] or [address]:port; the form [host]
       turns off MX lookups.  If you specify multiple SMTP destinations,
       Postfix will try them in the specified order.

       Note: before Postfix 2.2, do not use the fallback_relay feature when
       relaying mail for a backup or primary MX domain. Mail would loop
       between the Postfix MX host and the fallback_relay host when the final
       destination is unavailable.

       ⊕      In main.cf specify "relay_transport = relay",

       ⊕      In master.cf specify "-o fallback_relay =" (i.e., empty) at the
              end of the relay entry.

       ⊕      In transport maps, specify "relay:nexthop..." as the right-hand
              side for backup or primary MX domain entries.

       Postfix version 2.2 and later will not use the fallback_relay feature
       for destinations that it is MX host for.

fallback_transport (default: empty)
       Optional message delivery transport that the local(8) delivery agent
       should use for names that are not found in the aliases(5) or UNIX
       password database.

       The precedence of local(8) delivery features from high to low is:
       aliases, .forward files, mailbox_transport_maps, mailbox_transport,
       mailbox_command_maps, mailbox_command, home_mailbox,
       mail_spool_directory, fallback_transport_maps, fallback_transport and
       luser_relay.

fallback_transport_maps (default: empty)
       Optional lookup tables with per-recipient message delivery transports
       for recipients that the local(8) delivery agent could not find in the
       aliases(5) or UNIX password database.

       The precedence of local(8) delivery features from high to low is:
       aliases, .forward files, mailbox_transport_maps, mailbox_transport,
       mailbox_command_maps, mailbox_command, home_mailbox,
       mail_spool_directory, fallback_transport_maps, fallback_transport and
       luser_relay.

       For safety reasons, this feature does not allow $number substitutions
       in regular expression maps.

       This feature is available in Postfix 2.3 and later.

fast_flush_domains (default: $relay_domains)
       Optional list of destinations that are eligible for per-destination
       logfiles with mail that is queued to those destinations.

       By default, Postfix maintains "fast flush" logfiles only for
       destinations that the Postfix SMTP server is willing to relay to (i.e.
       the default is: "fast_flush_domains = $relay_domains"; see the
       relay_domains parameter in the postconf(5) manual).

       Specify a list of hosts or domains, "/file/name" patterns or
       "type:table" lookup tables, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace. A
       "/file/name" pattern is replaced by its contents; a "type:table" lookup
       table is matched when the domain or its parent domain appears as lookup
       key.

       Pattern matching of domain names is controlled by the presence or
       absence of "fast_flush_domains" in the parent_domain_matches_subdomains
       parameter value.

       Specify "fast_flush_domains =" (i.e., empty) to disable the feature
       altogether.

fast_flush_purge_time (default: 7d)
       The time after which an empty per-destination "fast flush" logfile is
       deleted.

       You can specify the time as a number, or as a number followed by a
       letter that indicates the time unit: s=seconds, m=minutes, h=hours,
       d=days, w=weeks.  The default time unit is days.

fast_flush_refresh_time (default: 12h)
       The time after which a non-empty but unread per-destination "fast
       flush" logfile needs to be refreshed.  The contents of a logfile are
       refreshed by requesting delivery of all messages listed in the logfile.

       You can specify the time as a number, or as a number followed by a
       letter that indicates the time unit: s=seconds, m=minutes, h=hours,
       d=days, w=weeks.  The default time unit is hours.

fault_injection_code (default: 0)
       Force specific internal tests to fail, to test the handling of errors
       that are difficult to reproduce otherwise.

flush_service_name (default: flush)
       The name of the flush(8) service. This service maintains
       per-destination logfiles with the queue file names of mail that is
       queued for those destinations.

       This feature is available in Postfix 2.0 and later.

fork_attempts (default: 5)
       The maximal number of attempts to fork() a child process.

fork_delay (default: 1s)
       The delay between attempts to fork() a child process.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

forward_expansion_filter (default: see postconf -d output)
       Restrict the characters that the local(8) delivery agent allows in
       $name expansions of $forward_path.  Characters outside the allowed set
       are replaced by underscores.

forward_path (default: see postconf -d output)
       The local(8) delivery agent search list for finding a .forward file
       with user-specified delivery methods. The first file that is found is
       used.

       The forward_path value is not subject to Postfix configuration
       parameter $name expansion. Instead, the following $name expansions are
       done on forward_path before the search actually happens.  The result of
       $name expansion is filtered with the character set that is specified
       with the forward_expansion_filter parameter.

       $user  The recipient's username.

       $shell The recipient's login shell pathname.

       $home  The recipient's home directory.

       $recipient
              The full recipient address.

       $extension
              The optional recipient address extension.

       $domain
              The recipient domain.

       $local The entire recipient localpart.

       $recipient_delimiter
              The address extension delimiter that was found in the recipient
              address (Postfix 2.11 and later), or the system-wide recipient
              address extension delimiter (Postfix 2.10 and earlier).

       ${name?value}

       ${name?{value}} (Postfix >= 3.0)
              Expands to value when $name is non-empty.

       ${name:value}

       ${name:{value}} (Postfix >= 3.0)
              Expands to value when $name is empty.

       ${name?{value1}:{value2}} (Postfix >= 3.0)
              Expands to value1 when $name is non-empty, value2 otherwise.

       Instead of $name you can also specify ${name} or $(name).

       Examples:

       forward_path = /var/forward/$user
       forward_path =
           /var/forward/$user/.forward$recipient_delimiter$extension,
           /var/forward/$user/.forward

frozen_delivered_to (default: yes)
       Update the local(8) delivery agent's idea of the Delivered-To: address
       (see prepend_delivered_header) only once, at the start of a delivery
       attempt; do not update the Delivered-To: address while expanding
       aliases or .forward files.

       This feature is available in Postfix 2.3 and later. With older Postfix
       releases, the behavior is as if this parameter is set to "no". The old
       setting can be expensive with deeply nested aliases or .forward files.
       When an alias or .forward file changes the Delivered-To: address, it
       ties up one queue file and one cleanup process instance while mail is
       being forwarded.

hash_queue_depth (default: 1)
       The number of subdirectory levels for queue directories listed with the
       hash_queue_names parameter. Queue hashing is implemented by creating
       one or more levels of directories with one-character names.
       Originally, these directory names were equal to the first characters of
       the queue file name, with the hexadecimal representation of the file
       creation time in microseconds.

       With long queue file names, queue hashing produces the same results as
       with short names. The file creation time in microseconds is converted
       into hexadecimal form before the result is used for queue hashing.  The
       base 16 encoding gives finer control over the number of subdirectories
       than is possible with the base 52 encoding of long queue file names.

       After changing the hash_queue_names or hash_queue_depth parameter,
       execute the command "postfix reload".

hash_queue_names (default: deferred, defer)
       The names of queue directories that are split across multiple
       subdirectory levels.

       Before Postfix version 2.2, the default list of hashed queues was
       significantly larger. Claims about improvements in file system
       technology suggest that hashing of the incoming and active queues is no
       longer needed. Fewer hashed directories speed up the time needed to
       restart Postfix.

       After changing the hash_queue_names or hash_queue_depth parameter,
       execute the command "postfix reload".

header_address_token_limit (default: 10240)
       The maximal number of address tokens are allowed in an address message
       header. Information that exceeds the limit is discarded.  The limit is
       enforced by the cleanup(8) server.

header_checks (default: empty)
       Optional lookup tables for content inspection of primary non-MIME
       message headers, as specified in the header_checks(5) manual page.

header_from_format (default: standard)
       The format of the Postfix-generated From: header. This setting affects
       the appearance of 'full name' information when a local program such as
       /bin/mail submits a message without a From: header through the Postfix
       sendmail(1) command.

       Specify one of the following:

       standard (default)
              Produce a header formatted as "From: name <address>".  This is
              the default as of Postfix 3.3.

       obsolete
              Produce a header formatted as "From: address (name)". This is
              the behavior prior to Postfix 3.3.

       Notes:

       ⊕      Postfix generates the format "From: address" when name
              information is unavailable or the envelope sender address is
              empty. This is the same behavior as prior to Postfix 3.3.

       ⊕      In the standard form, the name will be quoted if it contains
              specials as defined in RFC 5322, or the "!%" address operators.

       ⊕      The Postfix sendmail(1) command gets name information from the
              -F command-line option, from the NAME environment variable, or
              from the UNIX password file.

       This feature is available in Postfix 3.3 and later.

header_size_limit (default: 102400)
       The maximal amount of memory in bytes for storing a message header.  If
       a header is larger, the excess is discarded.  The limit is enforced by
       the cleanup(8) server.

helpful_warnings (default: yes)
       Log warnings about problematic configuration settings, and provide
       helpful suggestions.

       This feature is available in Postfix 2.0 and later.

home_mailbox (default: empty)
       Optional pathname of a mailbox file relative to a local(8) user's home
       directory.

       Specify a pathname ending in "/" for qmail-style delivery.

       The precedence of local(8) delivery features from high to low is:
       aliases, .forward files, mailbox_transport_maps, mailbox_transport,
       mailbox_command_maps, mailbox_command, home_mailbox,
       mail_spool_directory, fallback_transport_maps, fallback_transport and
       luser_relay.

       Examples:

       home_mailbox = Mailbox
       home_mailbox = Maildir/

hopcount_limit (default: 50)
       The maximal number of Received:  message headers that is allowed in the
       primary message headers. A message that exceeds the limit is bounced,
       in order to stop a mailer loop.

html_directory (default: see postconf -d output)
       The location of Postfix HTML files that describe how to build,
       configure or operate a specific Postfix subsystem or feature.

ignore_mx_lookup_error (default: no)
       Ignore DNS MX lookups that produce no response.  By default, the
       Postfix SMTP client defers delivery and tries again after some delay.
       This behavior is required by the SMTP standard.

       Specify "ignore_mx_lookup_error = yes" to force a DNS A record lookup
       instead. This violates the SMTP standard and can result in mis-delivery
       of mail.

import_environment (default: see postconf -d output)
       The list of environment variables that a privileged Postfix process
       will import from a non-Postfix parent process, or name=value
       environment overrides.  Unprivileged utilities will enforce the
       name=value overrides, but otherwise will not change their process
       environment.  Examples of relevant environment variables:

       TZ     May be needed for sane time keeping on most System-V-ish
              systems.

       DISPLAY
              Needed for debugging Postfix daemons with an X-windows debugger.

       XAUTHORITY
              Needed for debugging Postfix daemons with an X-windows debugger.

       MAIL_CONFIG
              Needed to make "postfix -c" work.

       Specify a list of names and/or name=value pairs, separated by
       whitespace or comma. Specify "{ name=value }" to protect whitespace or
       comma in environment variable values (whitespace after the opening "{"
       and before the closing "}" is ignored). The form name=value is
       supported with Postfix version 2.1 and later; the use of {} is
       supported with Postfix 3.0 and later.

in_flow_delay (default: 1s)
       Time to pause before accepting a new message, when the message arrival
       rate exceeds the message delivery rate. This feature is turned on by
       default (it's disabled on SCO UNIX due to an SCO bug).

       With the default 100 Postfix SMTP server process limit, "in_flow_delay
       = 1s" limits the mail inflow to 100 messages per second above the
       number of messages delivered per second.

       Specify 0 to disable the feature. Valid delays are 0..10.

inet_interfaces (default: all)
       The network interface addresses that this mail system receives mail on.
       Specify "all" to receive mail on all network interfaces (default), and
       "loopback-only" to receive mail on loopback network interfaces only
       (Postfix version 2.2 and later).  The parameter also controls delivery
       of mail to user@[ip.address].

       Note 1: you need to stop and start Postfix when this parameter changes.

       Note 2: address information may be enclosed inside [], but this form is
       not required here.

       When inet_interfaces specifies just one IPv4 and/or IPv6 address that
       is not a loopback address, the Postfix SMTP client will use this
       address as the IP source address for outbound mail. Support for IPv6 is
       available in Postfix version 2.2 and later.

       On a multi-homed firewall with separate Postfix instances listening on
       the "inside" and "outside" interfaces, this can prevent each instance
       from being able to reach remote SMTP servers on the "other side" of the
       firewall. Setting smtp_bind_address to 0.0.0.0 avoids the potential
       problem for IPv4, and setting smtp_bind_address6 to :: solves the
       problem for IPv6.

       A better solution for multi-homed firewalls is to leave inet_interfaces
       at the default value and instead use explicit IP addresses in the
       master.cf SMTP server definitions.  This preserves the Postfix SMTP
       client's loop detection, by ensuring that each side of the firewall
       knows that the other IP address is still the same host. Setting
       $inet_interfaces to a single IPv4 and/or IPV6 address is primarily
       useful with virtual hosting of domains on secondary IP addresses, when
       each IP address serves a different domain (and has a different
       $myhostname setting).

       See also the proxy_interfaces parameter, for network addresses that are
       forwarded to Postfix by way of a proxy or address translator.

       Examples:

       inet_interfaces = all (DEFAULT)
       inet_interfaces = loopback-only (Postfix version 2.2 and later)
       inet_interfaces = 127.0.0.1
       inet_interfaces = 127.0.0.1, [::1] (Postfix version 2.2 and later)
       inet_interfaces = 192.168.1.2, 127.0.0.1

inet_protocols (default: see 'postconf -d output')
       The Internet protocols Postfix will attempt to use when making or
       accepting connections. Specify one or more of "ipv4" or "ipv6",
       separated by whitespace or commas. The form "all" is equivalent to
       "ipv4, ipv6" or "ipv4", depending on whether the operating system
       implements IPv6.

       With Postfix 2.8 and earlier the default is "ipv4". For backwards
       compatibility with these releases, the Postfix 2.9 and later upgrade
       procedure appends an explicit "inet_protocols = ipv4" setting to
       main.cf when no explicit setting is present. This compatibility
       workaround will be phased out as IPv6 deployment becomes more common.

       This feature is available in Postfix 2.2 and later.

       Note: you MUST stop and start Postfix after changing this parameter.

       On systems that pre-date IPV6_V6ONLY support (RFC 3493), an IPv6 server
       will also accept IPv4 connections, even when IPv4 is turned off with
       the inet_protocols parameter.  On systems with IPV6_V6ONLY support,
       Postfix will use separate server sockets for IPv6 and IPv4, and each
       will accept only connections for the corresponding protocol.

       When IPv4 support is enabled via the inet_protocols parameter, Postfix
       will look up DNS type A records, and will convert IPv4-in-IPv6 client
       IP addresses (::ffff:1.2.3.4) to their original IPv4 form (1.2.3.4).
       The latter is needed on hosts that pre-date IPV6_V6ONLY support (RFC
       3493).

       When IPv6 support is enabled via the inet_protocols parameter, Postfix
       will do DNS type AAAA record lookups.

       When both IPv4 and IPv6 support are enabled, the Postfix SMTP client
       will choose the protocol as specified with the smtp_address_preference
       parameter. Postfix versions before 2.8 attempt to connect via IPv6
       before attempting to use IPv4.

       Examples:

       inet_protocols = ipv4
       inet_protocols = all (DEFAULT)
       inet_protocols = ipv6
       inet_protocols = ipv4, ipv6

info_log_address_format (default: external)
       The email address form that will be used in non-debug logging (info,
       warning, etc.). As of Postfix 3.5 when an address localpart contains
       spaces or other special characters, the localpart will be quoted, for
       example:

               from=<"name with spaces"@example.com>

       Older Postfix versions would log the internal (unquoted) form:

               from=<name with spaces@example.com>

       The external and internal forms are identical for the vast majority of
       email addresses that contain no spaces or other special characters in
       the localpart.

       The logging in external form is consistent with the address form that
       Postfix 3.2 and later prefer for most table lookups. This is therefore
       the more useful form for non-debug logging.

       Specify "info_log_address_format = internal" for backwards
       compatibility.

       Postfix uses the unquoted form internally, because an attacker can
       specify an email address in different forms by playing games with
       quotes and backslashes. An attacker should not be able to use such
       games to circumvent Postfix access policies.

       This feature is available in Postfix 3.5 and later.

initial_destination_concurrency (default: 5)
       The initial per-destination concurrency level for parallel delivery to
       the same destination.  With per-destination recipient limit > 1, a
       destination is a domain, otherwise it is a recipient.

       Use transport_initial_destination_concurrency to specify a
       transport-specific override, where transport is the master.cf name of
       the message delivery transport (Postfix 2.5 and later).

       Warning: with concurrency of 1, one bad message can be enough to block
       all mail to a site.

internal_mail_filter_classes (default: empty)
       What categories of Postfix-generated mail are subject to before-queue
       content inspection by non_smtpd_milters, header_checks and body_checks.
       Specify zero or more of the following, separated by whitespace or
       comma.

       bounce Inspect the content of delivery status notifications.

       notify Inspect the content of postmaster notifications by the smtp(8)
              and smtpd(8) processes.

       NOTE: It's generally not safe to enable content inspection of
       Postfix-generated email messages. The user is warned.

       This feature is available in Postfix 2.3 and later.

invalid_hostname_reject_code (default: 501)
       The numerical Postfix SMTP server response code when the client HELO or
       EHLO command parameter is rejected by the reject_invalid_helo_hostname
       restriction.

       Do not change this unless you have a complete understanding of RFC
       5321.

ipc_idle (default: version dependent)
       The time after which a client closes an idle internal communication
       channel.  The purpose is to allow Postfix daemon processes to terminate
       voluntarily after they become idle. This is used, for example, by the
       Postfix address resolving and rewriting clients.

       With Postfix 2.4 the default value was reduced from 100s to 5s.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

ipc_timeout (default: 3600s)
       The time limit for sending or receiving information over an internal
       communication channel.  The purpose is to break out of deadlock
       situations. If the time limit is exceeded the software aborts with a
       fatal error.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

ipc_ttl (default: 1000s)
       The time after which a client closes an active internal communication
       channel.  The purpose is to allow Postfix daemon processes to terminate
       voluntarily after reaching their client limit.  This is used, for
       example, by the Postfix address resolving and rewriting clients.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

known_tcp_ports (default: lmtp=24, smtp=25, smtps=submissions=465,
       submission=587)
       Optional setting that avoids lookups in the services(5) database.  This
       feature was implemented to address inconsistencies in the name of the
       port "465" service. The ABNF is:

           known_tcp_ports = empty | name-to-port *("," name-to-port)
           name-to-port = 1*(service-name "=') port-number

       The comma is required. Whitespace is optional but it cannot appear
       inside a service name or port number.

       This feature is available in Postfix 3.6 and later.

line_length_limit (default: 2048)
       Upon input, long lines are chopped up into pieces of at most this
       length; upon delivery, long lines are reconstructed.

lmdb_map_size (default: 16777216)
       The initial OpenLDAP LMDB database size limit in bytes.  Each time a
       database becomes full, its size limit is doubled.

       This feature is available in Postfix 2.11 and later.

lmtp_address_preference (default: ipv6)
       The LMTP-specific version of the smtp_address_preference configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.8 and later.

lmtp_address_verify_target (default: rcpt)
       The LMTP-specific version of the smtp_address_verify_target
       configuration parameter.  See there for details.

       This feature is available in Postfix 3.0 and later.

lmtp_assume_final (default: no)
       When a remote LMTP server announces no DSN support, assume that the
       server performs final delivery, and send "delivered" delivery status
       notifications instead of "relayed". The default setting is backwards
       compatible to avoid the infinitesimal possibility of breaking existing
       LMTP-based content filters.

lmtp_balance_inet_protocols (default: yes)
       The LMTP-specific version of the smtp_balance_inet_protocols
       configuration parameter. See there for details.

       This feature is available in Postfix 3.3 and later.

lmtp_bind_address (default: empty)
       The LMTP-specific version of the smtp_bind_address configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_bind_address6 (default: empty)
       The LMTP-specific version of the smtp_bind_address6 configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_bind_address_enforce (default: empty)
       The LMTP-specific version of the smtp_bind_address_enforce
       configuration parameter. See there for details.

       This feature is available in Postfix 3.7 and later.

lmtp_body_checks (default: empty)
       The LMTP-specific version of the smtp_body_checks configuration
       parameter. See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_cache_connection (default: yes)
       Keep Postfix LMTP client connections open for up to $max_idle seconds.
       When the LMTP client receives a request for the same connection the
       connection is reused.

       This parameter is available in Postfix version 2.2 and earlier.  With
       Postfix version 2.3 and later, see lmtp_connection_cache_on_demand,
       lmtp_connection_cache_destinations, or
       lmtp_connection_reuse_time_limit.

       The effectiveness of cached connections will be determined by the
       number of remote LMTP servers in use, and the concurrency limit
       specified for the Postfix LMTP client. Cached connections are closed
       under any of the following conditions:

       ⊕      The Postfix LMTP client idle time limit is reached.  This limit
              is specified with the Postfix max_idle configuration parameter.

       ⊕      A delivery request specifies a different destination than the
              one currently cached.

       ⊕      The per-process limit on the number of delivery requests is
              reached.  This limit is specified with the Postfix max_use
              configuration parameter.

       ⊕      Upon the onset of another delivery request, the remote LMTP
              server associated with the current session does not respond to
              the RSET command.

       Most of these limitations have been with the Postfix connection cache
       that is shared among multiple LMTP client programs.

lmtp_cname_overrides_servername (default: yes)
       The LMTP-specific version of the smtp_cname_overrides_servername
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_connect_timeout (default: 0s)
       The Postfix LMTP client time limit for completing a TCP connection, or
       zero (use the operating system built-in time limit).  When no
       connection can be made within the deadline, the LMTP client tries the
       next address on the mail exchanger list.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       Example:

       lmtp_connect_timeout = 30s

lmtp_connection_cache_destinations (default: empty)
       The LMTP-specific version of the smtp_connection_cache_destinations
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_connection_cache_on_demand (default: yes)
       The LMTP-specific version of the smtp_connection_cache_on_demand
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_connection_cache_time_limit (default: 2s)
       The LMTP-specific version of the smtp_connection_cache_time_limit
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_connection_reuse_count_limit (default: 0)
       The LMTP-specific version of the smtp_connection_reuse_count_limit
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.11 and later.

lmtp_connection_reuse_time_limit (default: 300s)
       The LMTP-specific version of the smtp_connection_reuse_time_limit
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_data_done_timeout (default: 600s)
       The Postfix LMTP client time limit for sending the LMTP ".", and for
       receiving the remote LMTP server response.  When no response is
       received within the deadline, a warning is logged that the mail may be
       delivered multiple times.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

lmtp_data_init_timeout (default: 120s)
       The Postfix LMTP client time limit for sending the LMTP DATA command,
       and for receiving the remote LMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

lmtp_data_xfer_timeout (default: 180s)
       The Postfix LMTP client time limit for sending the LMTP message
       content.  When the connection stalls for more than
       $lmtp_data_xfer_timeout the LMTP client terminates the transfer.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

lmtp_defer_if_no_mx_address_found (default: no)
       The LMTP-specific version of the smtp_defer_if_no_mx_address_found
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_delivery_status_filter (default: empty)
       The LMTP-specific version of the smtp_delivery_status_filter
       configuration parameter.  See there for details.

       This feature is available in Postfix 3.0 and later.

lmtp_destination_concurrency_limit (default:
       $default_destination_concurrency_limit)
       The maximal number of parallel deliveries to the same destination via
       the lmtp message delivery transport. This limit is enforced by the
       queue manager. The message delivery transport name is the first field
       in the entry in the master.cf file.

lmtp_destination_recipient_limit (default:
       $default_destination_recipient_limit)
       The maximal number of recipients per message for the lmtp message
       delivery transport. This limit is enforced by the queue manager. The
       message delivery transport name is the first field in the entry in the
       master.cf file.

       Setting this parameter to a value of 1 changes the meaning of
       lmtp_destination_concurrency_limit from concurrency per domain into
       concurrency per recipient.

lmtp_discard_lhlo_keyword_address_maps (default: empty)
       Lookup tables, indexed by the remote LMTP server address, with case
       insensitive lists of LHLO keywords (pipelining, starttls, auth, etc.)
       that the Postfix LMTP client will ignore in the LHLO response from a
       remote LMTP server. See lmtp_discard_lhlo_keywords for details. The
       table is not indexed by hostname for consistency with
       smtpd_discard_ehlo_keyword_address_maps.

       This feature is available in Postfix 2.3 and later.

lmtp_discard_lhlo_keywords (default: empty)
       A case insensitive list of LHLO keywords (pipelining, starttls, auth,
       etc.) that the Postfix LMTP client will ignore in the LHLO response
       from a remote LMTP server.

       This feature is available in Postfix 2.3 and later.

       Notes:

       ⊕      Specify the silent-discard pseudo keyword to prevent this action
              from being logged.

       ⊕      Use the lmtp_discard_lhlo_keyword_address_maps feature to
              discard LHLO keywords selectively.

lmtp_dns_reply_filter (default: empty)
       Optional filter for Postfix LMTP client DNS lookup results.  See
       smtp_dns_reply_filter for details including an example.

       This feature is available in Postfix 3.0 and later.

lmtp_dns_resolver_options (default: empty)
       The LMTP-specific version of the smtp_dns_resolver_options
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.8 and later.

lmtp_dns_support_level (default: empty)
       The LMTP-specific version of the smtp_dns_support_level configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.11 and later.

lmtp_enforce_tls (default: no)
       The LMTP-specific version of the smtp_enforce_tls configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_fallback_relay (default: empty)
       Optional list of relay hosts for LMTP destinations that can't be found
       or that are unreachable.  In main.cf elements are separated by
       whitespace or commas.

       By default, mail is returned to the sender when a destination is not
       found, and delivery is deferred when a destination is unreachable.

       The fallback relays must be TCP destinations, specified without a
       leading "inet:" prefix.  Specify a host or host:port.  Since MX lookups
       do not apply with LMTP, there is no need to use the "[host]" or
       "[host]:port" forms.  If you specify multiple LMTP destinations,
       Postfix will try them in the specified order.

       This feature is available in Postfix 3.1 and later.

lmtp_generic_maps (default: empty)
       The LMTP-specific version of the smtp_generic_maps configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_header_checks (default: empty)
       The LMTP-specific version of the smtp_header_checks configuration
       parameter. See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_host_lookup (default: dns)
       The LMTP-specific version of the smtp_host_lookup configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_lhlo_name (default: $myhostname)
       The hostname to send in the LMTP LHLO command.

       The default value is the machine hostname.  Specify a hostname or
       [ip.add.re.ss] or [ip:v6:add:re::ss].

       This information can be specified in the main.cf file for all LMTP
       clients, or it can be specified in the master.cf file for a specific
       client, for example:

           /etc/postfix/master.cf:
               mylmtp ... lmtp -o lmtp_lhlo_name=foo.bar.com

       This feature is available in Postfix 2.3 and later.

lmtp_lhlo_timeout (default: 300s)
       The Postfix LMTP client time limit for sending the LHLO command, and
       for receiving the initial remote LMTP server response.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
       The default time unit is s (seconds).

lmtp_line_length_limit (default: 990)
       The LMTP-specific version of the smtp_line_length_limit configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_mail_timeout (default: 300s)
       The Postfix LMTP client time limit for sending the MAIL FROM command,
       and for receiving the remote LMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

lmtp_mime_header_checks (default: empty)
       The LMTP-specific version of the smtp_mime_header_checks configuration
       parameter. See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_min_data_rate (default: 500)
       The LMTP-specific version of the smtp_min_data_rate configuration
       parameter. See there for details.

       This feature is available in Postfix 3.7 and later.

lmtp_mx_address_limit (default: 5)
       The LMTP-specific version of the smtp_mx_address_limit configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_mx_session_limit (default: 2)
       The LMTP-specific version of the smtp_mx_session_limit configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_nested_header_checks (default: empty)
       The LMTP-specific version of the smtp_nested_header_checks
       configuration parameter. See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_per_record_deadline (default: no)
       The LMTP-specific version of the smtp_per_record_deadline configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.9 and later.

lmtp_per_request_deadline (default: no)
       The LMTP-specific version of the smtp_per_request_deadline
       configuration parameter. See there for details.

       This feature is available in Postfix 3.7 and later.

lmtp_pix_workaround_delay_time (default: 10s)
       The LMTP-specific version of the smtp_pix_workaround_delay_time
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_pix_workaround_maps (default: empty)
       The LMTP-specific version of the smtp_pix_workaround_maps configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.4 and later.

lmtp_pix_workaround_threshold_time (default: 500s)
       The LMTP-specific version of the smtp_pix_workaround_threshold_time
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_pix_workarounds (default: empty)
       The LMTP-specific version of the smtp_pix_workaround configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.4 and later.

lmtp_quit_timeout (default: 300s)
       The Postfix LMTP client time limit for sending the QUIT command, and
       for receiving the remote LMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

lmtp_quote_rfc821_envelope (default: yes)
       The LMTP-specific version of the smtp_quote_rfc821_envelope
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_randomize_addresses (default: yes)
       The LMTP-specific version of the smtp_randomize_addresses configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_rcpt_timeout (default: 300s)
       The Postfix LMTP client time limit for sending the RCPT TO command, and
       for receiving the remote LMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

lmtp_reply_filter (default: empty)
       The LMTP-specific version of the smtp_reply_filter configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.7 and later.

lmtp_rset_timeout (default: 20s)
       The Postfix LMTP client time limit for sending the RSET command, and
       for receiving the remote LMTP server response. The LMTP client sends
       RSET in order to finish a recipient address probe, or to verify that a
       cached connection is still alive.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

lmtp_sasl_auth_cache_name (default: empty)
       The LMTP-specific version of the smtp_sasl_auth_cache_name
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_sasl_auth_cache_time (default: 90d)
       The LMTP-specific version of the smtp_sasl_auth_cache_time
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_sasl_auth_enable (default: no)
       Enable SASL authentication in the Postfix LMTP client.

lmtp_sasl_auth_soft_bounce (default: yes)
       The LMTP-specific version of the smtp_sasl_auth_soft_bounce
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_sasl_mechanism_filter (default: empty)
       The LMTP-specific version of the smtp_sasl_mechanism_filter
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_sasl_password_maps (default: empty)
       Optional Postfix LMTP client lookup tables with one username:password
       entry per host or domain.  If a remote host or domain has no
       username:password entry, then the Postfix LMTP client will not attempt
       to authenticate to the remote host.

lmtp_sasl_path (default: empty)
       Implementation-specific information that is passed through to the SASL
       plug-in implementation that is selected with lmtp_sasl_type.  Typically
       this specifies the name of a configuration file or rendezvous point.

       This feature is available in Postfix 2.3 and later.

lmtp_sasl_security_options (default: noplaintext, noanonymous)
       SASL security options; as of Postfix 2.3 the list of available features
       depends on the SASL client implementation that is selected with
       lmtp_sasl_type.

       The following security features are defined for the cyrus client SASL
       implementation:

       noplaintext
              Disallow authentication methods that use plaintext passwords.

       noactive
              Disallow authentication methods that are vulnerable to
              non-dictionary active attacks.

       nodictionary
              Disallow authentication methods that are vulnerable to passive
              dictionary attacks.

       noanonymous
              Disallow anonymous logins.

       Example:

       lmtp_sasl_security_options = noplaintext

lmtp_sasl_tls_security_options (default: $lmtp_sasl_security_options)
       The LMTP-specific version of the smtp_sasl_tls_security_options
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_sasl_tls_verified_security_options (default:
       $lmtp_sasl_tls_security_options)
       The LMTP-specific version of the
       smtp_sasl_tls_verified_security_options configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_sasl_type (default: cyrus)
       The SASL plug-in type that the Postfix LMTP client should use for
       authentication.  The available types are listed with the "postconf -A"
       command.

       This feature is available in Postfix 2.3 and later.

lmtp_send_dummy_mail_auth (default: no)
       The LMTP-specific version of the smtp_send_dummy_mail_auth
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.9 and later.

lmtp_send_xforward_command (default: no)
       Send an XFORWARD command to the remote LMTP server when the LMTP LHLO
       server response announces XFORWARD support.  This allows an lmtp(8)
       delivery agent, used for content filter message injection, to forward
       the name, address, protocol and HELO name of the original client to the
       content filter and downstream LMTP server.  Before you change the value
       to yes, it is best to make sure that your content filter supports this
       command.

       This feature is available in Postfix 2.1 and later.

lmtp_sender_dependent_authentication (default: no)
       The LMTP-specific version of the smtp_sender_dependent_authentication
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_skip_5xx_greeting (default: yes)
       The LMTP-specific version of the smtp_skip_5xx_greeting configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_skip_quit_response (default: no)
       Wait for the response to the LMTP QUIT command.

lmtp_starttls_timeout (default: 300s)
       The LMTP-specific version of the smtp_starttls_timeout configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tcp_port (default: 24)
       The default TCP port that the Postfix LMTP client connects to.  Specify
       a symbolic name (see services(5)) or a numeric port.

lmtp_tls_CAfile (default: empty)
       The LMTP-specific version of the smtp_tls_CAfile configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_CApath (default: empty)
       The LMTP-specific version of the smtp_tls_CApath configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_block_early_mail_reply (default: empty)
       The LMTP-specific version of the smtp_tls_block_early_mail_reply
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.7 and later.

lmtp_tls_cert_file (default: empty)
       The LMTP-specific version of the smtp_tls_cert_file configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_chain_files (default: empty)
       The LMTP-specific version of the smtp_tls_chain_files configuration
       parameter. See there for details.

       This feature is available in Postfix 3.4 and later.

lmtp_tls_ciphers (default: medium)
       The LMTP-specific version of the smtp_tls_ciphers configuration
       parameter. See there for details.

       This feature is available in Postfix 2.6 and later.

lmtp_tls_connection_reuse (default: no)
       The LMTP-specific version of the smtp_tls_connection_reuse
       configuration parameter. See there for details.

       This feature is available in Postfix 3.4 and later.

lmtp_tls_dcert_file (default: empty)
       The LMTP-specific version of the smtp_tls_dcert_file configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_dkey_file (default: $lmtp_tls_dcert_file)
       The LMTP-specific version of the smtp_tls_dkey_file configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_eccert_file (default: empty)
       The LMTP-specific version of the smtp_tls_eccert_file configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.6 and later, when Postfix is
       compiled and linked with OpenSSL 1.0.0 or later.

lmtp_tls_eckey_file (default: empty)
       The LMTP-specific version of the smtp_tls_eckey_file configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.6 and later, when Postfix is
       compiled and linked with OpenSSL 1.0.0 or later.

lmtp_tls_enforce_peername (default: yes)
       The LMTP-specific version of the smtp_tls_enforce_peername
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_exclude_ciphers (default: empty)
       The LMTP-specific version of the smtp_tls_exclude_ciphers configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_fingerprint_cert_match (default: empty)
       The LMTP-specific version of the smtp_tls_fingerprint_cert_match
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_tls_fingerprint_digest (default: see postconf -d output)
       The LMTP-specific version of the smtp_tls_fingerprint_digest
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.5 and later.

lmtp_tls_force_insecure_host_tlsa_lookup (default: no)
       The LMTP-specific version of the
       smtp_tls_force_insecure_host_tlsa_lookup configuration parameter.  See
       there for details.

       This feature is available in Postfix 2.11 and later.

lmtp_tls_key_file (default: $lmtp_tls_cert_file)
       The LMTP-specific version of the smtp_tls_key_file configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_loglevel (default: 0)
       The LMTP-specific version of the smtp_tls_loglevel configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_ciphers (default: medium)
       The LMTP-specific version of the smtp_tls_mandatory_ciphers
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_exclude_ciphers (default: empty)
       The LMTP-specific version of the smtp_tls_mandatory_exclude_ciphers
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_mandatory_protocols (default: see postconf -d output)
       The LMTP-specific version of the smtp_tls_mandatory_protocols
       configuration parameter. See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_note_starttls_offer (default: no)
       The LMTP-specific version of the smtp_tls_note_starttls_offer
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_per_site (default: empty)
       The LMTP-specific version of the smtp_tls_per_site configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_policy_maps (default: empty)
       The LMTP-specific version of the smtp_tls_policy_maps configuration
       parameter. See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_protocols (default: see postconf -d output)
       The LMTP-specific version of the smtp_tls_protocols configuration
       parameter. See there for details.

       This feature is available in Postfix 2.6 and later.

lmtp_tls_scert_verifydepth (default: 9)
       The LMTP-specific version of the smtp_tls_scert_verifydepth
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_secure_cert_match (default: nexthop)
       The LMTP-specific version of the smtp_tls_secure_cert_match
       configuration parameter. See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_security_level (default: empty)
       The LMTP-specific version of the smtp_tls_security_level configuration
       parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_servername (default: empty)
       The LMTP-specific version of the smtp_tls_servername configuration
       parameter. See there for details.

       This feature is available in Postfix 3.4 and later.

lmtp_tls_session_cache_database (default: empty)
       The LMTP-specific version of the smtp_tls_session_cache_database
       configuration parameter. See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_session_cache_timeout (default: 3600s)
       The LMTP-specific version of the smtp_tls_session_cache_timeout
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_trust_anchor_file (default: empty)
       The LMTP-specific version of the smtp_tls_trust_anchor_file
       configuration parameter.  See there for details.

       This feature is available in Postfix 2.11 and later.

lmtp_tls_verify_cert_match (default: hostname)
       The LMTP-specific version of the smtp_tls_verify_cert_match
       configuration parameter. See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_tls_wrappermode (default: no)
       The LMTP-specific version of the smtp_tls_wrappermode configuration
       parameter. See there for details.

       This feature is available in Postfix 3.0 and later.

lmtp_use_tls (default: no)
       The LMTP-specific version of the smtp_use_tls configuration parameter.
       See there for details.

       This feature is available in Postfix 2.3 and later.

lmtp_xforward_timeout (default: 300s)
       The Postfix LMTP client time limit for sending the XFORWARD command,
       and for receiving the remote LMTP server response.

       In case of problems the client does NOT try the next address on the
       mail exchanger list.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

local_command_shell (default: empty)
       Optional shell program for local(8) delivery to non-Postfix commands.
       By default, non-Postfix commands are executed directly; commands are
       given to the default shell (typically, /bin/sh) only when they contain
       shell meta characters or shell built-in commands.

       "sendmail's restricted shell" (smrsh) is what most people will use in
       order to restrict what programs can be run from e.g. .forward files
       (smrsh is part of the Sendmail distribution).

       Note: when a shell program is specified, it is invoked even when the
       command contains no shell built-in commands or meta characters.

       Example:

       local_command_shell = /some/where/smrsh -c
       local_command_shell = /bin/bash -c

local_delivery_status_filter (default: $default_delivery_status_filter)
       Optional filter for the local(8) delivery agent to change the status
       code or explanatory text of successful or unsuccessful deliveries.  See
       default_delivery_status_filter for details.

       This feature is available in Postfix 3.0 and later.

local_destination_concurrency_limit (default: 2)
       The maximal number of parallel deliveries via the local mail delivery
       transport to the same recipient (when
       "local_destination_recipient_limit = 1") or the maximal number of
       parallel deliveries to the same local domain (when
       "local_destination_recipient_limit > 1"). This limit is enforced by the
       queue manager. The message delivery transport name is the first field
       in the entry in the master.cf file.

       A low limit of 2 is recommended, just in case someone has an expensive
       shell command in a .forward file or in an alias (e.g., a mailing list
       manager).  You don't want to run lots of those at the same time.

local_destination_recipient_limit (default: 1)
       The maximal number of recipients per message delivery via the local
       mail delivery transport. This limit is enforced by the queue manager.
       The message delivery transport name is the first field in the entry in
       the master.cf file.

       Setting this parameter to a value > 1 changes the meaning of
       local_destination_concurrency_limit from concurrency per recipient into
       concurrency per domain.

local_header_rewrite_clients (default: permit_inet_interfaces)
       Rewrite message header addresses in mail from these clients and update
       incomplete addresses with the domain name in $myorigin or $mydomain;
       either don't rewrite message headers from other clients at all, or
       rewrite message headers and update incomplete addresses with the domain
       specified in the remote_header_rewrite_domain parameter.

       See the append_at_myorigin and append_dot_mydomain parameters for
       details of how domain names are appended to incomplete addresses.

       Specify a list of zero or more of the following:

       permit_inet_interfaces
              Append the domain name in $myorigin or $mydomain when the client
              IP address matches $inet_interfaces. This is enabled by default.

       permit_mynetworks
              Append the domain name in $myorigin or $mydomain when the client
              IP address matches any network or network address listed in
              $mynetworks. This setting will not prevent remote mail header
              address rewriting when mail from a remote client is forwarded by
              a neighboring system.

       permit_sasl_authenticated
              Append the domain name in $myorigin or $mydomain when the client
              is successfully authenticated via the RFC 4954 (AUTH) protocol.

       permit_tls_clientcerts
              Append the domain name in $myorigin or $mydomain when the remote
              SMTP client TLS certificate fingerprint or public key
              fingerprint (Postfix 2.9 and later) is listed in
              $relay_clientcerts.  The fingerprint digest algorithm is
              configurable via the smtpd_tls_fingerprint_digest parameter
              (hard-coded as md5 prior to Postfix version 2.5).
              The default algorithm is sha256 with Postfix >= 3.6 and the
              compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
              the default algorithm is md5.  The best-practice algorithm is
              now sha256. Recent advances in hash function cryptanalysis have
              led to md5 and sha1 being deprecated in favor of sha256.
              However, as long as there are no known "second pre-image"
              attacks against the older algorithms, their use in this context,
              though not recommended, is still likely safe.

       permit_tls_all_clientcerts
              Append the domain name in $myorigin or $mydomain when the remote
              SMTP client TLS certificate is successfully verified, regardless
              of whether it is listed on the server, and regardless of the
              certifying authority.

       check_address_map type:table

       type:table
              Append the domain name in $myorigin or $mydomain when the client
              IP address matches the specified lookup table.  The lookup
              result is ignored, and no subnet lookup is done. This is
              suitable for, e.g., pop-before-smtp lookup tables.

       Examples:

       The Postfix < 2.2 backwards compatible setting: always rewrite message
       headers, and always append my own domain to incomplete header
       addresses.

           local_header_rewrite_clients = static:all

       The purist (and default) setting: rewrite headers only in mail from
       Postfix sendmail and in SMTP mail from this machine.

           local_header_rewrite_clients = permit_inet_interfaces

       The intermediate setting: rewrite header addresses and append $myorigin
       or $mydomain information only with mail from Postfix sendmail, from
       local clients, or from authorized SMTP clients.

       Note: this setting will not prevent remote mail header address
       rewriting when mail from a remote client is forwarded by a neighboring
       system.

           local_header_rewrite_clients = permit_mynetworks,
               permit_sasl_authenticated permit_tls_clientcerts
               check_address_map hash:/etc/postfix/pop-before-smtp

local_login_sender_maps (default: static:*)
       A list of lookup tables that are searched by the UNIX login name, and
       that return a list of allowed envelope sender patterns separated by
       space or comma. These sender patterns are enforced by the Postfix
       postdrop(1) command. The default is backwards-compatible: every user
       may specify any sender envelope address.

       When no UNIX login name is available, the postdrop(1) command will
       prepend "uid:" to the numerical UID and use that instead.

       This feature ignores address extensions in the user-specified envelope
       sender address.

       The following sender patterns are special; these cannot be used as part
       of a longer pattern.

        *     This pattern allows any envelope sender address.

        <>    This pattern allows the empty envelope sender address. See the
              empty_address_local_login_sender_maps_lookup_key configuration
              parameter.

        @domain
              This pattern allows an envelope sender address when the '@' and
              domain part match.

       Examples:

       /etc/postfix/main.cf:
           # Allow root and postfix full control, anyone else can only
           # send mail as themselves. Use "uid:" followed by the numerical
           # UID when the UID has no entry in the UNIX password file.
           local_login_sender_maps =
               inline:{ { root = * }, { postfix = * } },
               pcre:/etc/postfix/login_senders

       /etc/postfix/login_senders:
          # Allow both the bare username and the user@domain forms.
           /(.+)/ $1 $1@example.com

       This feature is available in Postfix 3.6 and later.

local_recipient_maps (default: proxy:unix:passwd.byname $alias_maps)
       Lookup tables with all names or addresses of local recipients: a
       recipient address is local when its domain matches $mydestination,
       $inet_interfaces or $proxy_interfaces.  Specify @domain as a wild-card
       for domains that do not have a valid recipient list.  Technically,
       tables listed with $local_recipient_maps are used as lists: Postfix
       needs to know only if a lookup string is found or not, but it does not
       use the result from table lookup.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       If this parameter is non-empty (the default), then the Postfix SMTP
       server will reject mail for unknown local users.

       To turn off local recipient checking in the Postfix SMTP server,
       specify "local_recipient_maps =" (i.e. empty).

       The default setting assumes that you use the default Postfix local
       delivery agent for local delivery. You need to update the
       local_recipient_maps setting if:

       ⊕      You redefine the local delivery agent in master.cf.

       ⊕      You redefine the "local_transport" setting in main.cf.

       ⊕      You use the "luser_relay", "mailbox_transport", or
              "fallback_transport" feature of the Postfix local(8) delivery
              agent.

       Details are described in the LOCAL_RECIPIENT_README file.

       Beware: if the Postfix SMTP server runs chrooted, you need to access
       the passwd file via the proxymap(8) service, in order to overcome
       chroot access restrictions. The alternative, maintaining a copy of the
       system password file in the chroot jail is not practical.

       Examples:

       local_recipient_maps =

local_transport (default: local:$myhostname)
       The default mail delivery transport and next-hop destination for final
       delivery to domains listed with mydestination, and for [ipaddress]
       destinations that match $inet_interfaces or $proxy_interfaces.  This
       information can be overruled with the transport(5) table.

       By default, local mail is delivered to the transport called "local",
       which is just the name of a service that is defined the master.cf file.

       Specify a string of the form transport:nexthop, where transport is the
       name of a mail delivery transport defined in master.cf.  The :nexthop
       destination is optional; its syntax is documented in the manual page of
       the corresponding delivery agent.

       Beware: if you override the default local delivery agent then you need
       to review the LOCAL_RECIPIENT_README document, otherwise the SMTP
       server may reject mail for local recipients.

luser_relay (default: empty)
       Optional catch-all destination for unknown local(8) recipients.  By
       default, mail for unknown recipients in domains that match
       $mydestination, $inet_interfaces or $proxy_interfaces is returned as
       undeliverable.

       The luser_relay value is not subject to Postfix configuration parameter
       $name expansion. Instead, the following $name expansions are done:

       $domain
              The recipient domain.

       $extension
              The recipient address extension.

       $home  The recipient's home directory.

       $local The entire recipient address localpart.

       $recipient
              The full recipient address.

       $recipient_delimiter
              The address extension delimiter that was found in the recipient
              address (Postfix 2.11 and later), or the system-wide recipient
              address extension delimiter (Postfix 2.10 and earlier).

       $shell The recipient's login shell.

       $user  The recipient username.

       ${name?value}

       ${name?{value}} (Postfix >= 3.0)
              Expands to value when $name is non-empty.

       ${name:value}

       ${name:{value}} (Postfix >= 3.0)
              Expands to value when $name is empty.

       ${name?{value1}:{value2}} (Postfix >= 3.0)
              Expands to value1 when $name is non-empty, value2 otherwise.

       Instead of $name you can also specify ${name} or $(name).

       Note: luser_relay works only for the Postfix local(8) delivery agent.

       Note: if you use this feature for accounts not in the UNIX password
       file, then you must specify "local_recipient_maps =" (i.e. empty) in
       the main.cf file, otherwise the Postfix SMTP server will reject mail
       for non-UNIX accounts with "User unknown in local recipient table".

       Examples:

       luser_relay = $user@other.host
       luser_relay = $local@other.host
       luser_relay = admin+$local

mail_name (default: Postfix)
       The mail system name that is displayed in Received: headers, in the
       SMTP greeting banner, and in bounced mail.

mail_owner (default: postfix)
       The UNIX system account that owns the Postfix queue and most Postfix
       daemon processes.  Specify the name of an unprivileged user account
       that does not share a user or group ID with other accounts, and that
       owns no other files or processes on the system.  In particular, don't
       specify nobody or daemon.  PLEASE USE A DEDICATED USER ID AND GROUP ID.

       When this parameter value is changed you need to re-run "postfix
       set-permissions" (with Postfix version 2.0 and earlier:
       "/etc/postfix/post-install set-permissions".

mail_release_date (default: see postconf -d output)
       The Postfix release date, in "YYYYMMDD" format.

mail_spool_directory (default: see postconf -d output)
       The directory where local(8) UNIX-style mailboxes are kept. The default
       setting depends on the system type. Specify a name ending in / for
       maildir-style delivery.

       Note: maildir delivery is done with the privileges of the recipient.
       If you use the mail_spool_directory setting for maildir style delivery,
       then you must create the top-level maildir directory in advance.
       Postfix will not create it.

       Examples:

       mail_spool_directory = /var/mail
       mail_spool_directory = /var/spool/mail

mail_version (default: see postconf -d output)
       The version of the mail system. Stable releases are named
       major.minor.patchlevel. Experimental releases also include the release
       date. The version string can be used in, for example, the SMTP greeting
       banner.

mailbox_command (default: empty)
       Optional external command that the local(8) delivery agent should use
       for mailbox delivery.  The command is run with the user ID and the
       primary group ID privileges of the recipient.  Exception: command
       delivery for root executes with $default_privs privileges.  This is not
       a problem, because 1) mail for root should always be aliased to a real
       user and 2) don't log in as root, use "su" instead.

       The following environment variables are exported to the command:

       CLIENT_ADDRESS
              Remote client network address. Available in Postfix version 2.2
              and later.

       CLIENT_HELO
              Remote client EHLO command parameter. Available in Postfix
              version 2.2 and later.

       CLIENT_HOSTNAME
              Remote client hostname. Available in Postfix version 2.2 and
              later.

       CLIENT_PROTOCOL
              Remote client protocol. Available in Postfix version 2.2 and
              later.

       DOMAIN The domain part of the recipient address.

       EXTENSION
              The optional address extension.

       HOME   The recipient home directory.

       LOCAL  The recipient address localpart.

       LOGNAME
              The recipient's username.

       ORIGINAL_RECIPIENT
              The entire recipient address, before any address rewriting or
              aliasing.

       RECIPIENT
              The full recipient address.

       SASL_METHOD
              SASL authentication method specified in the remote client AUTH
              command. Available in Postfix version 2.2 and later.

       SASL_SENDER
              SASL sender address specified in the remote client MAIL FROM
              command. Available in Postfix version 2.2 and later.

       SASL_USER
              SASL username specified in the remote client AUTH command.
              Available in Postfix version 2.2 and later.

       SENDER The full sender address.

       SHELL  The recipient's login shell.

       USER   The recipient username.

       Unlike other Postfix configuration parameters, the mailbox_command
       parameter is not subjected to $name substitutions. This is to make it
       easier to specify shell syntax (see example below).

       If you can, avoid shell meta characters because they will force Postfix
       to run an expensive shell process. If you're delivering via "procmail"
       then running a shell won't make a noticeable difference in the total
       cost.

       Note: if you use the mailbox_command feature to deliver mail
       system-wide, you must set up an alias that forwards mail for root to a
       real user.

       The precedence of local(8) delivery features from high to low is:
       aliases, .forward files, mailbox_transport_maps, mailbox_transport,
       mailbox_command_maps, mailbox_command, home_mailbox,
       mail_spool_directory, fallback_transport_maps, fallback_transport and
       luser_relay.

       Examples:

       mailbox_command = /some/where/procmail
       mailbox_command = /some/where/procmail -a "$EXTENSION"
       mailbox_command = /some/where/maildrop -d "$USER"
               -f "$SENDER" "$EXTENSION"

mailbox_command_maps (default: empty)
       Optional lookup tables with per-recipient external commands to use for
       local(8) mailbox delivery.  Behavior is as with mailbox_command.

       The precedence of local(8) delivery features from high to low is:
       aliases, .forward files, mailbox_transport_maps, mailbox_transport,
       mailbox_command_maps, mailbox_command, home_mailbox,
       mail_spool_directory, fallback_transport_maps, fallback_transport and
       luser_relay.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

mailbox_delivery_lock (default: see postconf -d output)
       How to lock a UNIX-style local(8) mailbox before attempting delivery.
       For a list of available file locking methods, use the "postconf -l"
       command.

       This setting is ignored with maildir style delivery, because such
       deliveries are safe without explicit locks.

       Note: The dotlock method requires that the recipient UID or GID has
       write access to the parent directory of the mailbox file.

       Note: the default setting of this parameter is system dependent.

mailbox_size_limit (default: 51200000)
       The maximal size of any local(8) individual mailbox or maildir file, or
       zero (no limit).  In fact, this limits the size of any file that is
       written to upon local delivery, including files written by external
       commands that are executed by the local(8) delivery agent. The value
       cannot exceed LONG_MAX (typically, a 32-bit or 64-bit signed integer).

       This limit must not be smaller than the message size limit.

mailbox_transport (default: empty)
       Optional message delivery transport that the local(8) delivery agent
       should use for mailbox delivery to all local recipients, whether or not
       they are found in the UNIX passwd database.

       The precedence of local(8) delivery features from high to low is:
       aliases, .forward files, mailbox_transport_maps, mailbox_transport,
       mailbox_command_maps, mailbox_command, home_mailbox,
       mail_spool_directory, fallback_transport_maps, fallback_transport and
       luser_relay.

mailbox_transport_maps (default: empty)
       Optional lookup tables with per-recipient message delivery transports
       to use for local(8) mailbox delivery, whether or not the recipients are
       found in the UNIX passwd database.

       The precedence of local(8) delivery features from high to low is:
       aliases, .forward files, mailbox_transport_maps, mailbox_transport,
       mailbox_command_maps, mailbox_command, home_mailbox,
       mail_spool_directory, fallback_transport_maps, fallback_transport and
       luser_relay.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       For safety reasons, this feature does not allow $number substitutions
       in regular expression maps.

       This feature is available in Postfix 2.3 and later.

maillog_file (default: empty)
       The name of an optional logfile that is written by the Postfix
       postlogd(8) service. An empty value selects logging to syslogd(8).
       Specify "/dev/stdout" to select logging to standard output. Stdout
       logging requires that Postfix is started with "postfix start-fg".

       Note 1: The maillog_file parameter value must contain a prefix that is
       specified with the maillog_file_prefixes parameter.

       Note 2: Some Postfix non-daemon programs may still log information to
       syslogd(8), before they have processed their configuration parameters
       and command-line options.

       This feature is available in Postfix 3.4 and later.

maillog_file_compressor (default: gzip)
       The program to run after rotating $maillog_file with "postfix
       logrotate". The command is run with the rotated logfile name as its
       first argument.

       This feature is available in Postfix 3.4 and later.

maillog_file_prefixes (default: /var, /dev/stdout)
       A list of allowed prefixes for a maillog_file value. This is a safety
       feature to contain the damage from a single configuration mistake.
       Specify one or more prefix strings, separated by comma or whitespace.

       This feature is available in Postfix 3.4 and later.

maillog_file_rotate_suffix (default: %Y%m%d-%H%M%S)
       The format of the suffix to append to $maillog_file while rotating the
       file with "postfix logrotate". See strftime(3) for syntax. The default
       suffix, YYYYMMDD-HHMMSS, allows logs to be rotated frequently.

       This feature is available in Postfix 3.4 and later.

mailq_path (default: see postconf -d output)
       Sendmail compatibility feature that specifies where the Postfix
       mailq(1) command is installed. This command can be used to list the
       Postfix mail queue.

manpage_directory (default: see postconf -d output)
       Where the Postfix manual pages are installed.

maps_rbl_domains (default: empty)
       Obsolete feature: use the reject_rbl_client feature instead.

maps_rbl_reject_code (default: 554)
       The numerical Postfix SMTP server response code when a remote SMTP
       client request is blocked by the reject_rbl_client,
       reject_rhsbl_client, reject_rhsbl_reverse_client, reject_rhsbl_sender
       or reject_rhsbl_recipient restriction.

       Do not change this unless you have a complete understanding of RFC
       5321.

masquerade_classes (default: envelope_sender, header_sender, header_recipient)
       What addresses are subject to address masquerading.

       By default, address masquerading is limited to envelope sender
       addresses, and to header sender and header recipient addresses.  This
       allows you to use address masquerading on a mail gateway while still
       being able to forward mail to users on individual machines.

       Specify zero or more of: envelope_sender, envelope_recipient,
       header_sender, header_recipient

masquerade_domains (default: empty)
       Optional list of domains whose subdomain structure will be stripped off
       in email addresses.

       The list is processed left to right, and processing stops at the first
       match.  Thus,

           masquerade_domains = foo.example.com example.com

       strips "user@any.thing.foo.example.com" to "user@foo.example.com", but
       strips "user@any.thing.else.example.com" to "user@example.com".

       A domain name prefixed with ! means do not masquerade this domain or
       its subdomains. Thus,

           masquerade_domains = !foo.example.com example.com

       does not change "user@any.thing.foo.example.com" or
       "user@foo.example.com", but strips "user@any.thing.else.example.com" to
       "user@example.com".

       Note: with Postfix version 2.2, message header address masquerading
       happens only when message header address rewriting is enabled:

       ⊕      The message is received with the Postfix sendmail(1) command,

       ⊕      The message is received from a network client that matches
              $local_header_rewrite_clients,

       ⊕      The message is received from the network, and the
              remote_header_rewrite_domain parameter specifies a non-empty
              value.

       To get the behavior before Postfix version 2.2, specify
       "local_header_rewrite_clients = static:all".

       Example:

       masquerade_domains = $mydomain

masquerade_exceptions (default: empty)
       Optional list of user names that are not subjected to address
       masquerading, even when their addresses match $masquerade_domains.

       By default, address masquerading makes no exceptions.

       Specify a list of user names, "/file/name" or "type:table" patterns,
       separated by commas and/or whitespace. The list is matched left to
       right, and the search stops on the first match. A "/file/name" pattern
       is replaced by its contents; a "type:table" lookup table is matched
       when a name matches a lookup key (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude a name from the list. The form "!/file/name" is
       supported only in Postfix version 2.4 and later.

       Examples:

       masquerade_exceptions = root, mailer-daemon
       masquerade_exceptions = root

master_service_disable (default: empty)
       Selectively disable master(8) listener ports by service type or by
       service name and type.  Specify a list of service types ("inet",
       "unix", "fifo", or "pass") or "name/type" tuples, where "name" is the
       first field of a master.cf entry and "type" is a service type. As with
       other Postfix matchlists, a search stops at the first match.  Specify
       "!pattern" to exclude a service from the list. By default, all
       master(8) listener ports are enabled.

       Note: this feature does not support "/file/name" or "type:table"
       patterns, nor does it support wildcards such as "*" or "all". This is
       intentional.

       Examples:

       # With Postfix 2.6..2.10 use '.' instead of '/'.
       # Turn on all master(8) listener ports (the default).
       master_service_disable =
       # Turn off only the main SMTP listener port.
       master_service_disable = smtp/inet
       # Turn off all TCP/IP listener ports.
       master_service_disable = inet
       # Turn off all TCP/IP listener ports except "foo".
       master_service_disable = !foo/inet, inet

       This feature is available in Postfix 2.6 and later.

max_idle (default: 100s)
       The maximum amount of time that an idle Postfix daemon process waits
       for an incoming connection before terminating voluntarily.  This
       parameter is ignored by the Postfix queue manager and by other
       long-lived Postfix daemon processes.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

max_use (default: 100)
       The maximal number of incoming connections that a Postfix daemon
       process will service before terminating voluntarily.  This parameter is
       ignored by the Postfix queue manager and by other long-lived Postfix
       daemon processes.

maximal_backoff_time (default: 4000s)
       The maximal time between attempts to deliver a deferred message.

       This parameter should be set to a value greater than or equal to
       $minimal_backoff_time. See also $queue_run_delay.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

maximal_queue_lifetime (default: 5d)
       Consider a message as undeliverable, when delivery fails with a
       temporary error, and the time in the queue has reached the
       maximal_queue_lifetime limit.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       Specify 0 when mail delivery should be tried only once.

message_drop_headers (default: bcc, content-length, resent-bcc, return-path)
       Names of message headers that the cleanup(8) daemon will remove after
       applying header_checks(5) and before invoking Milter applications.  The
       default setting is compatible with Postfix < 3.0.

       Specify a list of header names, separated by comma or space.  Names are
       matched in a case-insensitive manner.  The list of supported header
       names is limited only by available memory.

       This feature is available in Postfix 3.0 and later.

message_reject_characters (default: empty)
       The set of characters that Postfix will reject in message content.  The
       usual C-like escape sequences are recognized: \a \b \f \n \r \t \v \ddd
       (up to three octal digits) and \\.

       Note 1: this feature does not recognize text that requires MIME
       decoding. It inspects raw message content, just like header_checks and
       body_checks.

       Note 2: this feature is disabled with "receive_override_options =
       no_header_body_checks".

       Example:

       message_reject_characters = \0

       This feature is available in Postfix 2.3 and later.

message_size_limit (default: 10240000)
       The maximal size in bytes of a message, including envelope information.
       The value cannot exceed LONG_MAX (typically, a 32-bit or 64-bit signed
       integer).

       Note: be careful when making changes.  Excessively small values will
       result in the loss of non-delivery notifications, when a bounce message
       size exceeds the local or remote MTA's message size limit.

message_strip_characters (default: empty)
       The set of characters that Postfix will remove from message content.
       The usual C-like escape sequences are recognized: \a \b \f \n \r \t \v
       \ddd (up to three octal digits) and \\.

       Note 1: this feature does not recognize text that requires MIME
       decoding. It inspects raw message content, just like header_checks and
       body_checks.

       Note 2: this feature is disabled with "receive_override_options =
       no_header_body_checks".

       Example:

       message_strip_characters = \0

       This feature is available in Postfix 2.3 and later.

meta_directory (default: see 'postconf -d' output)
       The location of non-executable files that are shared among multiple
       Postfix instances, such as postfix-files, dynamicmaps.cf, and the
       multi-instance template files main.cf.proto and master.cf.proto.  This
       directory should contain only Postfix-related files.  Typically, the
       meta_directory parameter has the same default as the config_directory
       parameter (/etc/postfix or /usr/local/etc/postfix).

       For backwards compatibility with Postfix versions 2.6..2.11, specify
       "meta_directory = $daemon_directory" in main.cf before installing or
       upgrading Postfix, or specify "meta_directory = /path/name" on the
       "make makefiles", "make install" or "make upgrade" command line.

       This feature is available in Postfix 3.0 and later.

milter_command_timeout (default: 30s)
       The time limit for sending an SMTP command to a Milter (mail filter)
       application, and for receiving the response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.3 and later.

milter_connect_macros (default: see postconf -d output)
       The macros that are sent to Milter (mail filter) applications after
       completion of an SMTP connection. See MILTER_README for a list of
       available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

milter_connect_timeout (default: 30s)
       The time limit for connecting to a Milter (mail filter) application,
       and for negotiating protocol options.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.3 and later.

milter_content_timeout (default: 300s)
       The time limit for sending message content to a Milter (mail filter)
       application, and for receiving the response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.3 and later.

milter_data_macros (default: see postconf -d output)
       The macros that are sent to version 4 or higher Milter (mail filter)
       applications after the SMTP DATA command. See MILTER_README for a list
       of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

milter_default_action (default: tempfail)
       The default action when a Milter (mail filter) response is unavailable
       (for example, bad Postfix configuration or Milter failure). Specify one
       of the following:

       accept Proceed as if the mail filter was not present.

       reject Reject all further commands in this session with a permanent
              status code.

       tempfail
              Reject all further commands in this session with a temporary
              status code.

       quarantine
              Like "accept", but freeze the message in the "hold" queue.
              Available with Postfix 2.6 and later.

       This feature is available in Postfix 2.3 and later.

milter_end_of_data_macros (default: see postconf -d output)
       The macros that are sent to Milter (mail filter) applications after the
       message end-of-data. See MILTER_README for a list of available macro
       names and their meanings.

       This feature is available in Postfix 2.3 and later.

milter_end_of_header_macros (default: see postconf -d output)
       The macros that are sent to Milter (mail filter) applications after the
       end of the message header. See MILTER_README for a list of available
       macro names and their meanings.

       This feature is available in Postfix 2.5 and later.

milter_header_checks (default: empty)
       Optional lookup tables for content inspection of message headers that
       are produced by Milter applications.  See the header_checks(5) manual
       page available actions. Currently, PREPEND is not implemented.

       The following example sends all mail that is marked as SPAM to a spam
       handling machine. Note that matches are case-insensitive by default.

       /etc/postfix/main.cf:
           milter_header_checks = pcre:/etc/postfix/milter_header_checks

       /etc/postfix/milter_header_checks:
           /^X-SPAM-FLAG:\s+YES/ FILTER mysmtp:sanitizer.example.com:25

       The milter_header_checks mechanism could also be used for allowlisting.
       For example it could be used to skip heavy content inspection for
       DKIM-signed mail from known friendly domains.

       This feature is available in Postfix 2.7, and as an optional patch for
       Postfix 2.6.

milter_helo_macros (default: see postconf -d output)
       The macros that are sent to Milter (mail filter) applications after the
       SMTP HELO or EHLO command. See MILTER_README for a list of available
       macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

milter_macro_daemon_name (default: $myhostname)
       The {daemon_name} macro value for Milter (mail filter) applications.
       See MILTER_README for a list of available macro names and their
       meanings.

       This feature is available in Postfix 2.3 and later.

milter_macro_defaults (default: empty)
       Optional list of name=value pairs that specify default values for
       arbitrary macros that Postfix may send to Milter applications.  These
       defaults are used when there is no corresponding information from the
       message delivery context.

       Specify name=value or {name=value} pairs separated by comma or
       whitespace.  Enclose a pair in "{}" when a value contains comma or
       whitespace (this form ignores whitespace after the enclosing "{",
       around the "=", and before the enclosing "}").

       This feature is available in Postfix 3.1 and later.

milter_macro_v (default: $mail_name $mail_version)
       The {v} macro value for Milter (mail filter) applications.  See
       MILTER_README for a list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

milter_mail_macros (default: see postconf -d output)
       The macros that are sent to Milter (mail filter) applications after the
       SMTP MAIL FROM command. See MILTER_README for a list of available macro
       names and their meanings.

       This feature is available in Postfix 2.3 and later.

milter_protocol (default: 6)
       The mail filter protocol version and optional protocol extensions for
       communication with a Milter application; prior to Postfix 2.6 the
       default protocol is 2. Postfix sends this version number during the
       initial protocol handshake.  It should match the version number that is
       expected by the mail filter application (or by its Milter library).

       Protocol versions:

       2      Use Sendmail 8 mail filter protocol version 2 (default with
              Sendmail version 8.11 .. 8.13 and Postfix version 2.3 ..  2.5).

       3      Use Sendmail 8 mail filter protocol version 3.

       4      Use Sendmail 8 mail filter protocol version 4.

       6      Use Sendmail 8 mail filter protocol version 6 (default with
              Sendmail version 8.14 and Postfix version 2.6).

       Protocol extensions:

       no_header_reply
              Specify this when the Milter application will not reply for each
              individual message header.

       This feature is available in Postfix 2.3 and later.

milter_rcpt_macros (default: see postconf -d output)
       The macros that are sent to Milter (mail filter) applications after the
       SMTP RCPT TO command. See MILTER_README for a list of available macro
       names and their meanings.

       This feature is available in Postfix 2.3 and later.

milter_unknown_command_macros (default: see postconf -d output)
       The macros that are sent to version 3 or higher Milter (mail filter)
       applications after an unknown SMTP command.  See MILTER_README for a
       list of available macro names and their meanings.

       This feature is available in Postfix 2.3 and later.

mime_boundary_length_limit (default: 2048)
       The maximal length of MIME multipart boundary strings. The MIME
       processor is unable to distinguish between boundary strings that do not
       differ in the first $mime_boundary_length_limit characters.

       This feature is available in Postfix 2.0 and later.

mime_header_checks (default: $header_checks)
       Optional lookup tables for content inspection of MIME related message
       headers, as described in the header_checks(5) manual page.

       This feature is available in Postfix 2.0 and later.

mime_nesting_limit (default: 100)
       The maximal recursion level that the MIME processor will handle.
       Postfix refuses mail that is nested deeper than the specified limit.

       This feature is available in Postfix 2.0 and later.

minimal_backoff_time (default: 300s)
       The minimal time between attempts to deliver a deferred message; prior
       to Postfix 2.4 the default value was 1000s.

       This parameter also limits the time an unreachable destination is kept
       in the short-term, in-memory, destination status cache.

       This parameter should be set greater than or equal to $queue_run_delay.
       See also $maximal_backoff_time.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

multi_instance_directories (default: empty)
       An optional list of non-default Postfix configuration directories;
       these directories belong to additional Postfix instances that share the
       Postfix executable files and documentation with the default Postfix
       instance, and that are started, stopped, etc., together with the
       default Postfix instance.  Specify a list of pathnames separated by
       comma or whitespace.

       When $multi_instance_directories is empty, the postfix(1) command runs
       in single-instance mode and operates on a single Postfix instance only.
       Otherwise, the postfix(1) command runs in multi-instance mode and
       invokes the multi-instance manager specified with the
       multi_instance_wrapper parameter. The multi-instance manager in turn
       executes postfix(1) commands for the default instance and for all
       Postfix instances in $multi_instance_directories.

       Currently, this parameter setting is ignored except for the default
       main.cf file.

       This feature is available in Postfix 2.6 and later.

multi_instance_enable (default: no)
       Allow this Postfix instance to be started, stopped, etc., by a
       multi-instance manager.  By default, new instances are created in a
       safe state that prevents them from being started inadvertently.  This
       parameter is reserved for the multi-instance manager.

       This feature is available in Postfix 2.6 and later.

multi_instance_group (default: empty)
       The optional instance group name of this Postfix instance. A group
       identifies closely-related Postfix instances that the multi-instance
       manager can start, stop, etc., as a unit.  This parameter is reserved
       for the multi-instance manager.

       This feature is available in Postfix 2.6 and later.

multi_instance_name (default: empty)
       The optional instance name of this Postfix instance. This name becomes
       also the default value for the syslog_name parameter.

       This feature is available in Postfix 2.6 and later.

multi_instance_wrapper (default: empty)
       The pathname of a multi-instance manager command that the postfix(1)
       command invokes when the multi_instance_directories parameter value is
       non-empty. The pathname may be followed by initial command arguments
       separated by whitespace; shell metacharacters such as quotes are not
       supported in this context.

       The postfix(1) command invokes the manager command with the postfix(1)
       non-option command arguments on the manager command line, and with all
       installation configuration parameters exported into the manager command
       process environment. The manager command in turn invokes the postfix(1)
       command for individual Postfix instances as "postfix -c
       config_directory command".

       This feature is available in Postfix 2.6 and later.

multi_recipient_bounce_reject_code (default: 550)
       The numerical Postfix SMTP server response code when a remote SMTP
       client request is blocked by the reject_multi_recipient_bounce
       restriction.

       Do not change this unless you have a complete understanding of RFC
       5321.

       This feature is available in Postfix 2.1 and later.

mydestination (default: $myhostname, localhost.$mydomain, localhost)
       The list of domains that are delivered via the $local_transport mail
       delivery transport. By default this is the Postfix local(8) delivery
       agent which looks up all recipients in /etc/passwd and /etc/aliases.
       The SMTP server validates recipient addresses with
       $local_recipient_maps and rejects non-existent recipients. See also the
       local domain class in the ADDRESS_CLASS_README file.

       The default mydestination value specifies names for the local machine
       only.  On a mail domain gateway, you should also include $mydomain.

       The $local_transport delivery method is also selected for mail
       addressed to user@[the.net.work.address] of the mail system (the IP
       addresses specified with the inet_interfaces and proxy_interfaces
       parameters).

       Warnings:

       ⊕      Do not specify the names of virtual domains - those domains are
              specified elsewhere. See VIRTUAL_README for more information.

       ⊕      Do not specify the names of domains that this machine is backup
              MX host for. See STANDARD_CONFIGURATION_README for how to set up
              backup MX hosts.

       ⊕      By default, the Postfix SMTP server rejects mail for recipients
              not listed with the local_recipient_maps parameter.  See the
              postconf(5) manual for a description of the local_recipient_maps
              and unknown_local_recipient_reject_code parameters.

       Specify a list of host or domain names, "/file/name" or "type:table"
       patterns, separated by commas and/or whitespace. A "/file/name" pattern
       is replaced by its contents; a "type:table" lookup table is matched
       when a name matches a lookup key (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace.

       Examples:

       mydestination = $myhostname, localhost.$mydomain $mydomain
       mydestination = $myhostname, localhost.$mydomain www.$mydomain, ftp.$mydomain

mydomain (default: see postconf -d output)
       The internet domain name of this mail system.  The default is to use
       $myhostname minus the first component, or "localdomain" (Postfix 2.3
       and later).  $mydomain is used as a default value for many other
       configuration parameters.

       Example:

       mydomain = domain.tld

myhostname (default: see postconf -d output)
       The internet hostname of this mail system. The default is to use the
       fully-qualified domain name (FQDN) from gethostname(), or to use the
       non-FQDN result from gethostname() and append ".$mydomain".
       $myhostname is used as a default value for many other configuration
       parameters.

       Example:

       myhostname = host.example.com

mynetworks (default: see postconf -d output)
       The list of "trusted" remote SMTP clients that have more privileges
       than "strangers".

       In particular, "trusted" SMTP clients are allowed to relay mail through
       Postfix.  See the smtpd_relay_restrictions parameter description in the
       postconf(5) manual.

       You can specify the list of "trusted" network addresses by hand or you
       can let Postfix do it for you (which is the default).  See the
       description of the mynetworks_style parameter for more information.

       If you specify the mynetworks list by hand, Postfix ignores the
       mynetworks_style setting.

       Specify a list of network addresses or network/netmask patterns,
       separated by commas and/or whitespace. Continue long lines by starting
       the next line with whitespace.

       The netmask specifies the number of bits in the network part of a host
       address.  You can also specify "/file/name" or "type:table" patterns.
       A "/file/name" pattern is replaced by its contents; a "type:table"
       lookup table is matched when a table entry matches a lookup string (the
       lookup result is ignored).

       The list is matched left to right, and the search stops on the first
       match.  Specify "!pattern" to exclude an address or network block from
       the list. The form "!/file/name" is supported only in Postfix version
       2.4 and later.

       Note 1: Pattern matching of domain names is controlled by the presence
       or absence of "mynetworks" in the parent_domain_matches_subdomains
       parameter value.

       Note 2: IP version 6 address information must be specified inside [] in
       the mynetworks value, and in files specified with "/file/name".  IP
       version 6 addresses contain the ":" character, and would otherwise be
       confused with a "type:table" pattern.

       Note 3: CIDR ranges cannot be specified in hash tables.  Use cidr
       tables if CIDR ranges are used.

       Examples:

       mynetworks = 127.0.0.0/8 168.100.189.0/28
       mynetworks = !192.168.0.1, 192.168.0.0/28
       mynetworks = 127.0.0.0/8 168.100.189.0/28 [::1]/128 [2001:240:587::]/64
       mynetworks = $config_directory/mynetworks
       mynetworks = hash:/etc/postfix/network_table
       mynetworks = cidr:/etc/postfix/network_table.cidr

mynetworks_style (default: Postfix >= 3.0: host, Postfix < 3.0: subnet)
       The method to generate the default value for the mynetworks parameter.
       This is the list of trusted networks for relay access control etc.

       ⊕      Specify "mynetworks_style = host" when Postfix should "trust"
              only the local machine.

       ⊕      Specify "mynetworks_style = subnet" when Postfix should "trust"
              remote SMTP clients in the same IP subnetworks as the local
              machine.  On Linux, this works correctly only with interfaces
              specified with the "ifconfig" or "ip" command.

       ⊕      Specify "mynetworks_style = class" when Postfix should "trust"
              remote SMTP clients in the same IP class A/B/C networks as the
              local machine.  Caution: this may cause Postfix to "trust" your
              entire provider's network.  Instead, specify an explicit
              mynetworks list by hand, as described with the mynetworks
              configuration parameter.

myorigin (default: $myhostname)
       The domain name that locally-posted mail appears to come from, and that
       locally posted mail is delivered to. The default, $myhostname, is
       adequate for small sites.  If you run a domain with multiple machines,
       you should (1) change this to $mydomain and (2) set up a domain-wide
       alias database that aliases each user to user@that.users.mailhost.

       Example:

       myorigin = $mydomain

nested_header_checks (default: $header_checks)
       Optional lookup tables for content inspection of non-MIME message
       headers in attached messages, as described in the header_checks(5)
       manual page.

       This feature is available in Postfix 2.0 and later.

newaliases_path (default: see postconf -d output)
       Sendmail compatibility feature that specifies the location of the
       newaliases(1) command. This command can be used to rebuild the local(8)
       aliases(5) database.

non_fqdn_reject_code (default: 504)
       The numerical Postfix SMTP server reply code when a client request is
       rejected by the reject_non_fqdn_helo_hostname, reject_non_fqdn_sender
       or reject_non_fqdn_recipient restriction.

non_smtpd_milters (default: empty)
       A list of Milter (mail filter) applications for new mail that does not
       arrive via the Postfix smtpd(8) server. This includes local submission
       via the sendmail(1) command line, new mail that arrives via the Postfix
       qmqpd(8) server, and old mail that is re-injected into the queue with
       "postsuper -r".  Specify space or comma as a separator. See the
       MILTER_README document for details.

       This feature is available in Postfix 2.3 and later.

notify_classes (default: resource, software)
       The list of error classes that are reported to the postmaster. These
       postmaster notifications do not replace user notifications. The default
       is to report only the most serious problems. The paranoid may wish to
       turn on the policy (UCE and mail relaying) and protocol error (broken
       mail software) reports.

       NOTE: postmaster notifications may contain confidential information
       such as SASL passwords or message content.  It is the system
       administrator's responsibility to treat such information with care.

       The error classes are:

       bounce (also implies 2bounce)
              Send the postmaster copies of the headers of bounced mail, and
              send transcripts of SMTP sessions when Postfix rejects mail. The
              notification is sent to the address specified with the
              bounce_notice_recipient configuration parameter (default:
              postmaster).

       2bounce
              Send undeliverable bounced mail to the postmaster. The
              notification is sent to the address specified with the
              2bounce_notice_recipient configuration parameter (default:
              postmaster).

       data   Send the postmaster a transcript of the SMTP session with an
              error because a critical data file was unavailable. The
              notification is sent to the address specified with the
              error_notice_recipient configuration parameter (default:
              postmaster).
              This feature is available in Postfix 2.9 and later.

       delay  Send the postmaster copies of the headers of delayed mail (see
              delay_warning_time). The notification is sent to the address
              specified with the delay_notice_recipient configuration
              parameter (default: postmaster).

       policy Send the postmaster a transcript of the SMTP session when a
              client request was rejected because of (UCE) policy. The
              notification is sent to the address specified with the
              error_notice_recipient configuration parameter (default:
              postmaster).

       protocol
              Send the postmaster a transcript of the SMTP session in case of
              client or server protocol errors. The notification is sent to
              the address specified with the error_notice_recipient
              configuration parameter (default: postmaster).

       resource
              Inform the postmaster of mail not delivered due to resource
              problems.  The notification is sent to the address specified
              with the error_notice_recipient configuration parameter
              (default: postmaster).

       software
              Inform the postmaster of mail not delivered due to software
              problems.  The notification is sent to the address specified
              with the error_notice_recipient configuration parameter
              (default: postmaster).

       Examples:

       notify_classes = bounce, delay, policy, protocol, resource, software
       notify_classes = 2bounce, resource, software

openssl_path (default: openssl)
       The location of the OpenSSL command line program openssl(1).  This is
       used by the "postfix tls" command to create private keys, certificate
       signing requests, self-signed certificates, and to compute public key
       digests for DANE TLSA records.  In multi-instance environments, this
       parameter is always determined from the configuration of the default
       Postfix instance.

       Example:

           /etc/postfix/main.cf:
               # NetBSD pkgsrc:
               openssl_path = /usr/pkg/bin/openssl
               # Local build:
               openssl_path = /usr/local/bin/openssl

       This feature is available in Postfix 3.1 and later.

owner_request_special (default: yes)
       Enable special treatment for owner-listname entries in the aliases(5)
       file, and don't split owner-listname and listname-request address
       localparts when the recipient_delimiter is set to "-".  This feature is
       useful for mailing lists.

parent_domain_matches_subdomains (default: see postconf -d output)
       A list of Postfix features where the pattern "example.com" also matches
       subdomains of example.com, instead of requiring an explicit
       ".example.com" pattern.  This is planned backwards compatibility:
       eventually, all Postfix features are expected to require explicit
       ".example.com" style patterns when you really want to match subdomains.

       The following Postfix feature names are supported.

       Postfix version 1.0 and later
              debug_peer_list, fast_flush_domains, mynetworks,
              permit_mx_backup_networks, relay_domains, transport_maps

       Postfix version 1.1 and later
              qmqpd_authorized_clients, smtpd_access_maps,

       Postfix version 2.8 and later
              postscreen_access_list

       Postfix version 3.0 and later
              smtpd_client_event_limit_exceptions

permit_mx_backup_networks (default: empty)
       Restrict the use of the permit_mx_backup SMTP access feature to only
       domains whose primary MX hosts match the listed networks.  The
       parameter value syntax is the same as with the mynetworks parameter;
       note, however, that the default value is empty.

       Pattern matching of domain names is controlled by the presence or
       absence of "permit_mx_backup_networks" in the
       parent_domain_matches_subdomains parameter value.

pickup_service_name (default: pickup)
       The name of the pickup(8) service. This service picks up local mail
       submissions from the Postfix maildrop queue.

       This feature is available in Postfix 2.0 and later.

pipe_delivery_status_filter (default: $default_delivery_status_filter)
       Optional filter for the pipe(8) delivery agent to change the delivery
       status code or explanatory text of successful or unsuccessful
       deliveries.  See default_delivery_status_filter for details.

       This feature is available in Postfix 3.0 and later.

plaintext_reject_code (default: 450)
       The numerical Postfix SMTP server response code when a request is
       rejected by the reject_plaintext_session restriction.

       This feature is available in Postfix 2.3 and later.

postlog_service_name (default: postlog)
       The name of the postlogd(8) service entry in master.cf.  This service
       appends logfile records to the file specified with the maillog_file
       parameter.

       This feature is available in Postfix 3.4 and later.

postlogd_watchdog_timeout (default: 10s)
       How much time a postlogd(8) process may take to process a request
       before it is terminated by a built-in watchdog timer. This is a safety
       mechanism that prevents postlogd(8) from becoming non-responsive due to
       a bug in Postfix itself or in system software. This limit cannot be set
       under 10s.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 3.4 and later.

postmulti_control_commands (default: reload flush)
       The postfix(1) commands that the postmulti(1) instance manager treats
       as "control" commands, that operate on running instances. For these
       commands, disabled instances are skipped.

       This feature is available in Postfix 2.6 and later.

postmulti_start_commands (default: start)
       The postfix(1) commands that the postmulti(1) instance manager treats
       as "start" commands. For these commands, disabled instances are
       "checked" rather than "started", and failure to "start" a member
       instance of an instance group will abort the start-up of later
       instances.

       This feature is available in Postfix 2.6 and later.

postmulti_stop_commands (default: see postconf -d output)
       The postfix(1) commands that the postmulti(1) instance manager treats
       as "stop" commands. For these commands, disabled instances are skipped,
       and enabled instances are processed in reverse order.

       This feature is available in Postfix 2.6 and later.

postscreen_access_list (default: permit_mynetworks)
       Permanent allow/denylist for remote SMTP client IP addresses.
       postscreen(8) searches this list immediately after a remote SMTP client
       connects.  Specify a comma- or whitespace-separated list of commands
       (in upper or lower case) or lookup tables. The search stops upon the
       first command that fires for the client IP address.

        permit_mynetworks
              Allowlist the client and terminate the search if the client IP
              address matches $mynetworks.  Do not subject the client to any
              before/after 220 greeting tests.  Pass the connection
              immediately to a Postfix SMTP server process.
              Pattern matching of domain names is controlled by the presence
              or absence of "postscreen_access_list" in the
              parent_domain_matches_subdomains parameter value.

        type:table
              Query the specified lookup table. Each table lookup result is an
              access list, except that access lists inside a table cannot
              specify type:table entries.
              To discourage the use of hash, btree, etc. tables, there is no
              support for substring matching like smtpd(8). Use CIDR tables
              instead.

        permit
              Allowlist the client and terminate the search. Do not subject
              the client to any before/after 220 greeting tests. Pass the
              connection immediately to a Postfix SMTP server process.

        reject
              Denylist the client and terminate the search. Subject the client
              to the action configured with the postscreen_denylist_action
              configuration parameter.

        dunno All postscreen(8) access lists implicitly have this command at
              the end.
              When  dunno  is executed inside a lookup table, return from the
              lookup table and evaluate the next command.
              When  dunno  is executed outside a lookup table, terminate the
              search, and subject the client to the configured before/after
              220 greeting tests.

       Example:

       /etc/postfix/main.cf:
           postscreen_access_list = permit_mynetworks,
               cidr:/etc/postfix/postscreen_access.cidr
           # Postfix < 3.6 use postscreen_blacklist_action.
           postscreen_denylist_action = enforce

       /etc/postfix/postscreen_access.cidr:
           # Rules are evaluated in the order as specified.
           # Denylist 192.168.* except 192.168.0.1.
           192.168.0.1         dunno
           192.168.0.0/16      reject

       This feature is available in Postfix 2.8.

postscreen_allowlist_interfaces (default: static:all)
       A list of local postscreen(8) server IP addresses where a
       non-allowlisted remote SMTP client can obtain postscreen(8)'s temporary
       allowlist status. This status is required before the client can talk to
       a Postfix SMTP server process.  By default, a client can obtain
       postscreen(8)'s allowlist status on any local postscreen(8) server IP
       address.

       When postscreen(8) listens on both primary and backup MX addresses, the
       postscreen_allowlist_interfaces parameter can be configured to give the
       temporary allowlist status only when a client connects to a primary MX
       address. Once a client is allowlisted it can talk to a Postfix SMTP
       server on any address. Thus, clients that connect only to backup MX
       addresses will never become allowlisted, and will never be allowed to
       talk to a Postfix SMTP server process.

       Specify a list of network addresses or network/netmask patterns,
       separated by commas and/or whitespace. The netmask specifies the number
       of bits in the network part of a host address. Continue long lines by
       starting the next line with whitespace.

       You can also specify "/file/name" or "type:table" patterns.  A
       "/file/name" pattern is replaced by its contents; a "type:table" lookup
       table is matched when a table entry matches a lookup string (the lookup
       result is ignored).

       The list is matched left to right, and the search stops on the first
       match. Specify "!pattern" to exclude an address or network block from
       the list.

       Note: IP version 6 address information must be specified inside [] in
       the postscreen_allowlist_interfaces value, and in files specified with
       "/file/name".  IP version 6 addresses contain the ":" character, and
       would otherwise be confused with a "type:table" pattern.

       Example:

       /etc/postfix/main.cf:
           # Don't allowlist connections to the backup IP address.
           # Postfix < 3.6 use postscreen_whitelist_interfaces.
           postscreen_allowlist_interfaces = !168.100.189.8, static:all

       This feature is available in Postfix 3.6 and later.

       Available as postscreen_whitelist_interfaces in Postfix 2.9 - 3.5.

postscreen_bare_newline_action (default: ignore)
       The action that postscreen(8) takes when a remote SMTP client sends a
       bare newline character, that is, a newline not preceded by carriage
       return.  Specify one of the following:

       ignore Ignore the failure of this test. Allow other tests to complete.
              Do not repeat this test before the result from some other test
              expires.  This option is useful for testing and collecting
              statistics without blocking mail permanently.

       enforce
              Allow other tests to complete. Reject attempts to deliver mail
              with a 550 SMTP reply, and log the helo/sender/recipient
              information.  Repeat this test the next time the client
              connects.

       drop   Drop the connection immediately with a 521 SMTP reply. Repeat
              this test the next time the client connects.

       This feature is available in Postfix 2.8.

postscreen_bare_newline_enable (default: no)
       Enable "bare newline" SMTP protocol tests in the postscreen(8) server.
       These tests are expensive: a remote SMTP client must disconnect after
       it passes the test, before it can talk to a real Postfix SMTP server.

       This feature is available in Postfix 2.8.

postscreen_bare_newline_ttl (default: 30d)
       The amount of time that postscreen(8) will use the result from a
       successful "bare newline" SMTP protocol test. During this time, the
       client IP address is excluded from this test. The default is long
       because a remote SMTP client must disconnect after it passes the test,
       before it can talk to a real Postfix SMTP server.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.8.

postscreen_blacklist_action (default: ignore)
       Renamed to postscreen_denylist_action in Postfix 3.6.

       This feature is available in Postfix 2.8 - 3.5.

postscreen_cache_cleanup_interval (default: 12h)
       The amount of time between postscreen(8) cache cleanup runs.  Cache
       cleanup increases the load on the cache database and should therefore
       not be run frequently. This feature requires that the cache database
       supports the "delete" and "sequence" operators.  Specify a zero
       interval to disable cache cleanup.

       After each cache cleanup run, the postscreen(8) daemon logs the number
       of entries that were retained and dropped. A cleanup run is logged as
       "partial" when the daemon terminates early after "postfix reload",
       "postfix stop", or no requests for $max_idle seconds.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is h (hours).

       This feature is available in Postfix 2.8.

postscreen_cache_map (default: btree:$data_directory/postscreen_cache)
       Persistent storage for the postscreen(8) server decisions.

       To share a postscreen(8) cache between multiple postscreen(8)
       instances, use "postscreen_cache_map = proxy:btree:/path/to/file".
       This requires Postfix version 2.9 or later; earlier proxymap(8)
       implementations don't support cache cleanup. For an alternative
       approach see the memcache_table(5) manpage.

       This feature is available in Postfix 2.8.

postscreen_cache_retention_time (default: 7d)
       The amount of time that postscreen(8) will cache an expired temporary
       allowlist entry before it is removed. This prevents clients from being
       logged as "NEW" just because their cache entry expired an hour ago. It
       also prevents the cache from filling up with clients that passed some
       deep protocol test once and never came back.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.8.

postscreen_client_connection_count_limit (default:
       $smtpd_client_connection_count_limit)
       How many simultaneous connections any remote SMTP client is allowed to
       have with the postscreen(8) daemon. By default, this limit is the same
       as with the Postfix SMTP server. Note that the triage process can take
       several seconds, with the time spent in postscreen_greet_wait delay,
       and with the time spent talking to the postscreen(8) built-in dummy
       SMTP protocol engine.

       This feature is available in Postfix 2.8.

postscreen_command_count_limit (default: 20)
       The limit on the total number of commands per SMTP session for
       postscreen(8)'s built-in SMTP protocol engine.  This SMTP engine defers
       or rejects all attempts to deliver mail, therefore there is no need to
       enforce separate limits on the number of junk commands and error
       commands.

       This feature is available in Postfix 2.8.

postscreen_command_filter (default: $smtpd_command_filter)
       A mechanism to transform commands from remote SMTP clients.  See
       smtpd_command_filter for further details.

       This feature is available in Postfix 2.8 and later.

postscreen_command_time_limit (default: normal: 300s, overload: 10s)
       The time limit to read an entire command line with postscreen(8)'s
       built-in SMTP protocol engine.

       This feature is available in Postfix 2.8.

postscreen_denylist_action (default: ignore)
       The action that postscreen(8) takes when a remote SMTP client is
       permanently denylisted with the postscreen_access_list parameter.
       Specify one of the following:

       ignore (default)
              Ignore  this result. Allow other tests to complete.  Repeat this
              test the next time the client connects.  This option is useful
              for testing and collecting statistics without blocking mail.

       enforce
              Allow other tests to complete. Reject attempts to deliver mail
              with a 550 SMTP reply, and log the helo/sender/recipient
              information.  Repeat this test the next time the client
              connects.

       drop   Drop the connection immediately with a 521 SMTP reply. Repeat
              this test the next time the client connects.

       This feature is available in Postfix 3.6 and later.

       Available as postscreen_blacklist_action in Postfix 2.8 - 3.5.

postscreen_disable_vrfy_command (default: $disable_vrfy_command)
       Disable the SMTP VRFY command in the postscreen(8) daemon.  See
       disable_vrfy_command for details.

       This feature is available in Postfix 2.8.

postscreen_discard_ehlo_keyword_address_maps (default:
       $smtpd_discard_ehlo_keyword_address_maps)
       Lookup tables, indexed by the remote SMTP client address, with case
       insensitive lists of EHLO keywords (pipelining, starttls, auth, etc.)
       that the postscreen(8) server will not send in the EHLO response to a
       remote SMTP client. See smtpd_discard_ehlo_keywords for details.  The
       table is not searched by hostname for robustness reasons.

       This feature is available in Postfix 2.8 and later.

postscreen_discard_ehlo_keywords (default: $smtpd_discard_ehlo_keywords)
       A case insensitive list of EHLO keywords (pipelining, starttls, auth,
       etc.) that the postscreen(8) server will not send in the EHLO response
       to a remote SMTP client. See smtpd_discard_ehlo_keywords for details.

       This feature is available in Postfix 2.8 and later.

postscreen_dnsbl_action (default: ignore)
       The action that postscreen(8) takes when a remote SMTP client's
       combined DNSBL score is equal to or greater than a threshold (as
       defined with the postscreen_dnsbl_sites and postscreen_dnsbl_threshold
       parameters).  Specify one of the following:

       ignore (default)
              Ignore the failure of this test. Allow other tests to complete.
              Repeat this test the next time the client connects.  This option
              is useful for testing and collecting statistics without blocking
              mail.

       enforce
              Allow other tests to complete. Reject attempts to deliver mail
              with a 550 SMTP reply, and log the helo/sender/recipient
              information.  Repeat this test the next time the client
              connects.

       drop   Drop the connection immediately with a 521 SMTP reply. Repeat
              this test the next time the client connects.

       This feature is available in Postfix 2.8.

postscreen_dnsbl_allowlist_threshold (default: 0)
       Allow a remote SMTP client to skip "before" and "after 220 greeting"
       protocol tests, based on its combined DNSBL score as defined with the
       postscreen_dnsbl_sites parameter.

       Specify a negative value to enable this feature. When a client passes
       the postscreen_dnsbl_allowlist_threshold without having failed other
       tests, all pending or disabled tests are flagged as completed with a
       time-to-live value equal to postscreen_dnsbl_ttl.  When a test was
       already completed, its time-to-live value is updated if it was less
       than postscreen_dnsbl_ttl.

       This feature is available in Postfix 3.6 and later.

       Available as postscreen_dnsbl_whitelist_threshold in Postfix 2.11 -
       3.5.

postscreen_dnsbl_max_ttl (default:
       ${postscreen_dnsbl_ttl?{$postscreen_dnsbl_ttl}:{1}}h)
       The maximum amount of time that postscreen(8) will use the result from
       a successful DNS-based reputation test before a client IP address is
       required to pass that test again. If the DNS reply specifies a shorter
       TTL value, that value will be used unless it would be smaller than
       postscreen_dnsbl_min_ttl.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is h (hours).

       This feature is available in Postfix 3.1. The default setting is
       backwards-compatible with older Postfix versions.

postscreen_dnsbl_min_ttl (default: 60s)
       The minimum amount of time that postscreen(8) will use the result from
       a successful DNS-based reputation test before a client IP address is
       required to pass that test again. If the DNS reply specifies a larger
       TTL value, that value will be used unless it would be larger than
       postscreen_dnsbl_max_ttl.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 3.1.

postscreen_dnsbl_reply_map (default: empty)
       A mapping from an actual DNSBL domain name which includes a secret
       password, to the DNSBL domain name that postscreen will reply with when
       it rejects mail.  When no mapping is found, the actual DNSBL domain
       will be used.

       For maximal stability it is best to use a file that is read into memory
       such as pcre:, regexp: or texthash: (texthash: is similar to hash:,
       except a) there is no need to run postmap(1) before the file can be
       used, and b) texthash: does not detect changes after the file is read).

       Example:

       /etc/postfix/main.cf:
           postscreen_dnsbl_reply_map = texthash:/etc/postfix/dnsbl_reply

       /etc/postfix/dnsbl_reply:
          secret.zen.spamhaus.org      zen.spamhaus.org

       This feature is available in Postfix 2.8.

postscreen_dnsbl_sites (default: empty)
       Optional list of DNS allow/denylist domains, filters and weight
       factors. When the list is non-empty, the dnsblog(8) daemon will query
       these domains with the IP addresses of remote SMTP clients, and
       postscreen(8) will update an SMTP client's DNSBL score with each
       non-error reply.

       Caution: when postscreen rejects mail, it replies with the DNSBL domain
       name. Use the postscreen_dnsbl_reply_map feature to hide "password"
       information in DNSBL domain names.

       When a client's score is equal to or greater than the threshold
       specified with postscreen_dnsbl_threshold, postscreen(8) can drop the
       connection with the remote SMTP client.

       Specify a list of domain=filter*weight entries, separated by comma or
       whitespace.

       ⊕      When no "=filter" is specified, postscreen(8) will use any
              non-error DNSBL reply.  Otherwise, postscreen(8) uses only DNSBL
              replies that match the filter. The filter has the form d.d.d.d,
              where each d is a number, or a pattern inside [] that contains
              one or more ";"-separated numbers or number..number ranges.

       ⊕      When no "*weight" is specified, postscreen(8) increments the
              remote SMTP client's DNSBL score by 1.  Otherwise, the weight
              must be an integral number, and postscreen(8) adds the specified
              weight to the remote SMTP client's DNSBL score.  Specify a
              negative number for allowlisting.

       ⊕      When one postscreen_dnsbl_sites entry produces multiple DNSBL
              responses, postscreen(8) applies the weight at most once.

       Examples:

       To use example.com as a high-confidence blocklist, and to block mail
       with example.net and example.org only when both agree:

       postscreen_dnsbl_threshold = 2
       postscreen_dnsbl_sites = example.com*2, example.net, example.org

       To filter only DNSBL replies containing 127.0.0.4:

       postscreen_dnsbl_sites = example.com=127.0.0.4

       This feature is available in Postfix 2.8.

postscreen_dnsbl_threshold (default: 1)
       The inclusive lower bound for blocking a remote SMTP client, based on
       its combined DNSBL score as defined with the postscreen_dnsbl_sites
       parameter.

       This feature is available in Postfix 2.8.

postscreen_dnsbl_timeout (default: 10s)
       The time limit for DNSBL or DNSWL lookups. This is separate from the
       timeouts in the dnsblog(8) daemon which are defined by system
       resolver(3) routines.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 3.0.

postscreen_dnsbl_ttl (default: 1h)
       The amount of time that postscreen(8) will use the result from a
       successful DNS-based reputation test before a client IP address is
       required to pass that test again.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is h (hours).

       This feature is available in Postfix 2.8-3.0. It was replaced by
       postscreen_dnsbl_max_ttl in Postfix 3.1.

postscreen_dnsbl_whitelist_threshold (default: 0)
       Renamed to postscreen_dnsbl_allowlist_threshold in Postfix 3.6.

       This feature is available in Postfix 2.11 - 3.5.

postscreen_enforce_tls (default: $smtpd_enforce_tls)
       Mandatory TLS: announce STARTTLS support to remote SMTP clients, and
       require that clients use TLS encryption.  See
       smtpd_postscreen_enforce_tls for details.

       This feature is available in Postfix 2.8 and later.  Preferably, use
       postscreen_tls_security_level instead.

postscreen_expansion_filter (default: see postconf -d output)
       List of characters that are permitted in postscreen_reject_footer
       attribute expansions.  See smtpd_expansion_filter for further details.

       This feature is available in Postfix 2.8 and later.

postscreen_forbidden_commands (default: $smtpd_forbidden_commands)
       List of commands that the postscreen(8) server considers in violation
       of the SMTP protocol. See smtpd_forbidden_commands for syntax, and
       postscreen_non_smtp_command_action for possible actions.

       This feature is available in Postfix 2.8.

postscreen_greet_action (default: ignore)
       The action that postscreen(8) takes when a remote SMTP client speaks
       before its turn within the time specified with the
       postscreen_greet_wait parameter.  Specify one of the following:

       ignore (default)
              Ignore the failure of this test. Allow other tests to complete.
              Repeat this test the next time the client connects.  This option
              is useful for testing and collecting statistics without blocking
              mail.

       enforce
              Allow other tests to complete. Reject attempts to deliver mail
              with a 550 SMTP reply, and log the helo/sender/recipient
              information.  Repeat this test the next time the client
              connects.

       drop   Drop the connection immediately with a 521 SMTP reply. Repeat
              this test the next time the client connects.

       In either case, postscreen(8) will not allowlist the remote SMTP client
       IP address.

       This feature is available in Postfix 2.8.

postscreen_greet_banner (default: $smtpd_banner)
       The text in the optional "220-text..." server response that
       postscreen(8) sends ahead of the real Postfix SMTP server's "220
       text..." response, in an attempt to confuse bad SMTP clients so that
       they speak before their turn (pre-greet).  Specify an empty value to
       disable this feature.

       This feature is available in Postfix 2.8.

postscreen_greet_ttl (default: 1d)
       The amount of time that postscreen(8) will use the result from a
       successful PREGREET test. During this time, the client IP address is
       excluded from this test. The default is relatively short, because a
       good client can immediately talk to a real Postfix SMTP server.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.8.

postscreen_greet_wait (default: normal: 6s, overload: 2s)
       The amount of time that postscreen(8) will wait for an SMTP client to
       send a command before its turn, and for DNS blocklist lookup results to
       arrive (default: up to 2 seconds under stress, up to 6 seconds
       otherwise).

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.8.

postscreen_helo_required (default: $smtpd_helo_required)
       Require that a remote SMTP client sends HELO or EHLO before commencing
       a MAIL transaction.

       This feature is available in Postfix 2.8.

postscreen_non_smtp_command_action (default: drop)
       The action that postscreen(8) takes when a remote SMTP client sends
       non-SMTP commands as specified with the postscreen_forbidden_commands
       parameter.  Specify one of the following:

       ignore Ignore the failure of this test. Allow other tests to complete.
              Do not repeat this test before the result from some other test
              expires.  This option is useful for testing and collecting
              statistics without blocking mail permanently.

       enforce
              Allow other tests to complete. Reject attempts to deliver mail
              with a 550 SMTP reply, and log the helo/sender/recipient
              information.  Repeat this test the next time the client
              connects.

       drop   Drop the connection immediately with a 521 SMTP reply. Repeat
              this test the next time the client connects. This action is the
              same as with the Postfix SMTP server's smtpd_forbidden_commands
              feature.

       This feature is available in Postfix 2.8.

postscreen_non_smtp_command_enable (default: no)
       Enable "non-SMTP command" tests in the postscreen(8) server. These
       tests are expensive: a client must disconnect after it passes the test,
       before it can talk to a real Postfix SMTP server.

       This feature is available in Postfix 2.8.

postscreen_non_smtp_command_ttl (default: 30d)
       The amount of time that postscreen(8) will use the result from a
       successful "non_smtp_command" SMTP protocol test. During this time, the
       client IP address is excluded from this test. The default is long
       because a client must disconnect after it passes the test, before it
       can talk to a real Postfix SMTP server.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.8.

postscreen_pipelining_action (default: enforce)
       The action that postscreen(8) takes when a remote SMTP client sends
       multiple commands instead of sending one command and waiting for the
       server to respond.  Specify one of the following:

       ignore Ignore the failure of this test. Allow other tests to complete.
              Do not repeat this test before the result from some other test
              expires.  This option is useful for testing and collecting
              statistics without blocking mail permanently.

       enforce
              Allow other tests to complete. Reject attempts to deliver mail
              with a 550 SMTP reply, and log the helo/sender/recipient
              information.  Repeat this test the next time the client
              connects.

       drop   Drop the connection immediately with a 521 SMTP reply. Repeat
              this test the next time the client connects.

       This feature is available in Postfix 2.8.

postscreen_pipelining_enable (default: no)
       Enable "pipelining" SMTP protocol tests in the postscreen(8) server.
       These tests are expensive: a good client must disconnect after it
       passes the test, before it can talk to a real Postfix SMTP server.

       This feature is available in Postfix 2.8.

postscreen_pipelining_ttl (default: 30d)
       The amount of time that postscreen(8) will use the result from a
       successful "pipelining" SMTP protocol test. During this time, the
       client IP address is excluded from this test. The default is long
       because a good client must disconnect after it passes the test, before
       it can talk to a real Postfix SMTP server.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.8.

postscreen_post_queue_limit (default: $default_process_limit)
       The number of clients that can be waiting for service from a real
       Postfix SMTP server process. When this queue is full, all clients will
       receive a 421 response.

       This feature is available in Postfix 2.8.

postscreen_pre_queue_limit (default: $default_process_limit)
       The number of non-allowlisted clients that can be waiting for a
       decision whether they will receive service from a real Postfix SMTP
       server process. When this queue is full, all non-allowlisted clients
       will receive a 421 response.

       This feature is available in Postfix 2.8.

postscreen_reject_footer (default: $smtpd_reject_footer)
       Optional information that is appended after a 4XX or 5XX postscreen(8)
       server response. See smtpd_reject_footer for further details.

       This feature is available in Postfix 2.8 and later.

postscreen_reject_footer_maps (default: $smtpd_reject_footer_maps)
       Optional lookup table for information that is appended after a 4XX or
       5XX postscreen(8) server response. See smtpd_reject_footer_maps for
       further details.

       This feature is available in Postfix 3.4 and later.

postscreen_tls_security_level (default: $smtpd_tls_security_level)
       The SMTP TLS security level for the postscreen(8) server; when a
       non-empty value is specified, this overrides the obsolete parameters
       postscreen_use_tls and postscreen_enforce_tls. See
       smtpd_tls_security_level for details.

       This feature is available in Postfix 2.8 and later.

postscreen_upstream_proxy_protocol (default: empty)
       The name of the proxy protocol used by an optional before-postscreen
       proxy agent. When a proxy agent is used, this protocol conveys local
       and remote address and port information. Specify
       "postscreen_upstream_proxy_protocol = haproxy" to enable the haproxy
       protocol; version 2 is supported with Postfix 3.5 and later.

       This feature is available in Postfix 2.10 and later.

postscreen_upstream_proxy_timeout (default: 5s)
       The time limit for the proxy protocol specified with the
       postscreen_upstream_proxy_protocol parameter.

       This feature is available in Postfix 2.10 and later.

postscreen_use_tls (default: $smtpd_use_tls)
       Opportunistic TLS: announce STARTTLS support to remote SMTP clients,
       but do not require that clients use TLS encryption.

       This feature is available in Postfix 2.8 and later.  Preferably, use
       postscreen_tls_security_level instead.

postscreen_watchdog_timeout (default: 10s)
       How much time a postscreen(8) process may take to respond to a remote
       SMTP client command or to perform a cache operation before it is
       terminated by a built-in watchdog timer.  This is a safety mechanism
       that prevents postscreen(8) from becoming non-responsive due to a bug
       in Postfix itself or in system software.  To avoid false alarms and
       unnecessary cache corruption this limit cannot be set under 10s.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.8.

postscreen_whitelist_interfaces (default: static:all)
       Renamed to postscreen_allowlist_interfaces in Postfix 3.6.

       This feature is available in Postfix 2.9 - 3.5.

prepend_delivered_header (default: command, file, forward)
       The message delivery contexts where the Postfix local(8) delivery agent
       prepends a Delivered-To:  message header with the address that the mail
       was delivered to. This information is used for mail delivery loop
       detection.

       By default, the Postfix local delivery agent prepends a Delivered-To:
       header when forwarding mail and when delivering to file (mailbox) and
       command. Turning off the Delivered-To: header when forwarding mail is
       not recommended.

       Specify zero or more of forward, file, or command.

       Example:

       prepend_delivered_header = forward

process_id (read-only)
       The process ID of a Postfix command or daemon process.

process_id_directory (default: pid)
       The location of Postfix PID files relative to $queue_directory.  This
       is a read-only parameter.

process_name (read-only)
       The process name of a Postfix command or daemon process.

propagate_unmatched_extensions (default: canonical, virtual)
       What address lookup tables copy an address extension from the lookup
       key to the lookup result.

       For example, with a virtual(5) mapping of "joe@example.com =>
       joe.user@example.net", the address "joe+foo@example.com" would rewrite
       to "joe.user+foo@example.net".

       Specify zero or more of canonical, virtual, alias, forward, include or
       generic. These cause address extension propagation with canonical(5),
       virtual(5), and aliases(5) maps, with local(8) .forward and :include:
       file lookups, and with smtp(8) generic maps, respectively.

       Note: enabling this feature for types other than canonical and virtual
       is likely to cause problems when mail is forwarded to other sites,
       especially with mail that is sent to a mailing list exploder address.

       Examples:

       propagate_unmatched_extensions = canonical, virtual, alias,
               forward, include
       propagate_unmatched_extensions = canonical, virtual

proxy_interfaces (default: empty)
       The network interface addresses that this mail system receives mail on
       by way of a proxy or network address translation unit.

       This feature is available in Postfix 2.0 and later.

       You must specify your "outside" proxy/NAT addresses when your system is
       a backup MX host for other domains, otherwise mail delivery loops will
       happen when the primary MX host is down.

       Example:

       proxy_interfaces = 1.2.3.4

proxy_read_maps (default: see postconf -d output)
       The lookup tables that the proxymap(8) server is allowed to access for
       the read-only service.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma.  Table references that don't begin with proxy: are ignored.

       This feature is available in Postfix 2.0 and later.

proxy_write_maps (default: see postconf -d output)
       The lookup tables that the proxymap(8) server is allowed to access for
       the read-write service. Postfix-owned local database files should be
       stored under the Postfix-owned data_directory.  Table references that
       don't begin with proxy: are ignored.

       This feature is available in Postfix 2.5 and later.

proxymap_service_name (default: proxymap)
       The name of the proxymap read-only table lookup service.  This service
       is normally implemented by the proxymap(8) daemon.

       This feature is available in Postfix 2.6 and later.

proxywrite_service_name (default: proxywrite)
       The name of the proxywrite read-write table lookup service.  This
       service is normally implemented by the proxymap(8) daemon.

       This feature is available in Postfix 2.6 and later.

qmgr_clog_warn_time (default: 300s)
       The minimal delay between warnings that a specific destination is
       clogging up the Postfix active queue. Specify 0 to disable.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is enabled with the helpful_warnings parameter.

       This feature is available in Postfix 2.0 and later.

qmgr_daemon_timeout (default: 1000s)
       How much time a Postfix queue manager process may take to handle a
       request before it is terminated by a built-in watchdog timer.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.8 and later.

qmgr_fudge_factor (default: 100)
       Obsolete feature: the percentage of delivery resources that a busy mail
       system will use up for delivery of a large mailing  list message.

       This feature exists only in the oqmgr(8) old queue manager. The current
       queue manager solves the problem in a better way.

qmgr_ipc_timeout (default: 60s)
       The time limit for the queue manager to send or receive information
       over an internal communication channel.  The purpose is to break out of
       deadlock situations. If the time limit is exceeded the software either
       retries or aborts the operation.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.8 and later.

qmgr_message_active_limit (default: 20000)
       The maximal number of messages in the active queue.

qmgr_message_recipient_limit (default: 20000)
       The maximal number of recipients held in memory by the Postfix queue
       manager, and the maximal size of the short-term, in-memory "dead"
       destination status cache.

qmgr_message_recipient_minimum (default: 10)
       The minimal number of in-memory recipients for any message. This takes
       priority over any other in-memory recipient limits (i.e., the global
       qmgr_message_recipient_limit and the per transport _recipient_limit) if
       necessary. The minimum value allowed for this parameter is 1.

qmqpd_authorized_clients (default: empty)
       What remote QMQP clients are allowed to connect to the Postfix QMQP
       server port.

       By default, no client is allowed to use the service. This is because
       the QMQP server will relay mail to any destination.

       Specify a list of client patterns. A list pattern specifies a host
       name, a domain name, an internet address, or a network/mask pattern,
       where the mask specifies the number of bits in the network part.  When
       a pattern specifies a file name, its contents are substituted for the
       file name; when a pattern is a "type:table" table specification, table
       lookup is used instead.

       Patterns are separated by whitespace and/or commas. In order to reverse
       the result, precede a pattern with an exclamation point (!). The form
       "!/file/name" is supported only in Postfix version 2.4 and later.

       Pattern matching of domain names is controlled by the presence or
       absence of "qmqpd_authorized_clients" in the
       parent_domain_matches_subdomains parameter value.

       Example:

       qmqpd_authorized_clients = !192.168.0.1, 192.168.0.0/24

qmqpd_client_port_logging (default: no)
       Enable logging of the remote QMQP client port in addition to the
       hostname and IP address. The logging format is "host[address]:port".

       This feature is available in Postfix 2.5 and later.

qmqpd_error_delay (default: 1s)
       How long the Postfix QMQP server will pause before sending a negative
       reply to the remote QMQP client. The purpose is to slow down confused
       or malicious clients.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

qmqpd_timeout (default: 300s)
       The time limit for sending or receiving information over the network.
       If a read or write operation blocks for more than $qmqpd_timeout
       seconds the Postfix QMQP server gives up and disconnects.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

queue_directory (default: see postconf -d output)
       The location of the Postfix top-level queue directory. This is the root
       directory of Postfix daemon processes that run chrooted.

queue_file_attribute_count_limit (default: 100)
       The maximal number of (name=value) attributes that may be stored in a
       Postfix queue file. The limit is enforced by the cleanup(8) server.

       This feature is available in Postfix 2.0 and later.

queue_minfree (default: 0)
       The minimal amount of free space in bytes in the queue file system that
       is needed to receive mail.  This is currently used by the Postfix SMTP
       server to decide if it will accept any mail at all.

       By default, the Postfix SMTP server rejects MAIL FROM commands when the
       amount of free space is less than 1.5*$message_size_limit (Postfix
       version 2.1 and later).  To specify a higher minimum free space limit,
       specify a queue_minfree value that is at least 1.5*$message_size_limit.

       With Postfix versions 2.0 and earlier, a queue_minfree value of zero
       means there is no minimum required amount of free space.

queue_run_delay (default: 300s)
       The time between deferred queue scans by the queue manager; prior to
       Postfix 2.4 the default value was 1000s.

       This parameter should be set less than or equal to
       $minimal_backoff_time. See also $maximal_backoff_time.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

queue_service_name (default: qmgr)
       The name of the qmgr(8) service. This service manages the Postfix queue
       and schedules delivery requests.

       This feature is available in Postfix 2.0 and later.

rbl_reply_maps (default: empty)
       Optional lookup tables with RBL response templates. The tables are
       indexed by the RBL domain name. By default, Postfix uses the default
       template as specified with the default_rbl_reply configuration
       parameter. See there for a discussion of the syntax of RBL reply
       templates.

       This feature is available in Postfix 2.0 and later.

readme_directory (default: see postconf -d output)
       The location of Postfix README files that describe how to build,
       configure or operate a specific Postfix subsystem or feature.

receive_override_options (default: empty)
       Enable or disable recipient validation, built-in content filtering, or
       address mapping. Typically, these are specified in master.cf as
       command-line arguments for the smtpd(8), qmqpd(8) or pickup(8) daemons.

       Specify zero or more of the following options.  The options override
       main.cf settings and are either implemented by smtpd(8), qmqpd(8), or
       pickup(8) themselves, or they are forwarded to the cleanup server.

       no_unknown_recipient_checks
              Do not try to reject unknown recipients (SMTP server only).
              This is typically specified AFTER an external content filter.

       no_address_mappings
              Disable canonical address mapping, virtual alias map expansion,
              address masquerading, and automatic BCC (blind carbon-copy)
              recipients. This is typically specified BEFORE an external
              content filter.

       no_header_body_checks
              Disable header/body_checks. This is typically specified AFTER an
              external content filter.

       no_milters
              Disable Milter (mail filter) applications. This is typically
              specified AFTER an external content filter.

       Note: when the "BEFORE content filter" receive_override_options setting
       is specified in the main.cf file, specify the "AFTER content filter"
       receive_override_options setting in master.cf (and vice versa).

       Examples:

       receive_override_options =
           no_unknown_recipient_checks, no_header_body_checks
       receive_override_options = no_address_mappings

       This feature is available in Postfix 2.1 and later.

recipient_bcc_maps (default: empty)
       Optional BCC (blind carbon-copy) address lookup tables, indexed by
       recipient address.  The BCC address (multiple results are not
       supported) is added when mail enters from outside of Postfix.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       The table search order is as follows:

       ⊕      Look up the "user+extension@domain.tld" address including the
              optional address extension.

       ⊕      Look up the "user@domain.tld" address without the optional
              address extension.

       ⊕      Look up the "user+extension" address local part when the
              recipient domain equals $myorigin, $mydestination,
              $inet_interfaces or $proxy_interfaces.

       ⊕      Look up the "user" address local part when the recipient domain
              equals $myorigin, $mydestination, $inet_interfaces or
              $proxy_interfaces.

       ⊕      Look up the "@domain.tld" part.

       Note: with Postfix 2.3 and later the BCC address is added as if it was
       specified with NOTIFY=NONE. The sender will not be notified when the
       BCC address is undeliverable, as long as all down-stream software
       implements RFC 3461.

       Note: with Postfix 2.2 and earlier the sender will unconditionally be
       notified when the BCC address is undeliverable.

       Note: automatic BCC recipients are produced only for new mail.  To
       avoid mailer loops, automatic BCC recipients are not generated after
       Postfix forwards mail internally, or after Postfix generates mail
       itself.

       Example:

       recipient_bcc_maps = hash:/etc/postfix/recipient_bcc

       After a change, run "postmap /etc/postfix/recipient_bcc".

       This feature is available in Postfix 2.1 and later.

recipient_canonical_classes (default: envelope_recipient, header_recipient)
       What addresses are subject to recipient_canonical_maps address mapping.
       By default, recipient_canonical_maps address mapping is applied to
       envelope recipient addresses, and to header recipient addresses.

       Specify one or more of: envelope_recipient, header_recipient

       This feature is available in Postfix 2.2 and later.

recipient_canonical_maps (default: empty)
       Optional address mapping lookup tables for envelope and header
       recipient addresses.  The table format and lookups are documented in
       canonical(5).

       Note: $recipient_canonical_maps is processed before $canonical_maps.

       Example:

       recipient_canonical_maps = hash:/etc/postfix/recipient_canonical

recipient_delimiter (default: empty)
       The set of characters that can separate an email address localpart,
       user name, or a .forward file name from its extension.  For example,
       with "recipient_delimiter = +", the software tries user+foo@example.com
       before trying user@example.com, user+foo before trying user, and
       .forward+foo before trying .forward.

       More formally, an email address localpart or user name is separated
       from its extension by the first character that matches the
       recipient_delimiter set. The delimiter character and extension may then
       be used to generate an extended .forward file name. This implementation
       recognizes one delimiter character and one extension per email address
       localpart or email address. With Postfix 2.10 and earlier, the
       recipient_delimiter specifies a single character.

       See canonical(5), local(8), relocated(5) and virtual(5) for the effects
       of recipient_delimiter on lookups in aliases, canonical, virtual, and
       relocated maps, and see the propagate_unmatched_extensions parameter
       for propagating an extension from one email address to another.

       When used in command_execution_directory, forward_path, or luser_relay,
       ${recipient_delimiter} is replaced with the actual recipient delimiter
       that was found in the recipient email address (Postfix 2.11 and later),
       or it is replaced with the main.cf recipient_delimiter parameter value
       (Postfix 2.10 and earlier).

       The recipient_delimiter is not applied to the mailer-daemon address,
       the postmaster address, or the double-bounce address. With the default
       "owner_request_special = yes" setting, the recipient_delimiter is also
       not applied to addresses with the special "owner-" prefix or the
       special "-request" suffix.

       Examples:

       # Handle Postfix-style extensions.
       recipient_delimiter = +

       # Handle both Postfix and qmail extensions (Postfix 2.11 and later).
       recipient_delimiter = +-

       # Use .forward for mail without address extension, and for mail with
       # an unrecognized address extension.
       forward_path = $home/.forward${recipient_delimiter}${extension},
           $home/.forward

reject_code (default: 554)
       The numerical Postfix SMTP server response code when a remote SMTP
       client request is rejected by the "reject" restriction.

       Do not change this unless you have a complete understanding of RFC
       5321.

reject_tempfail_action (default: defer_if_permit)
       The Postfix SMTP server's action when a reject-type restriction fails
       due to a temporary error condition. Specify "defer" to defer the remote
       SMTP client request immediately. With the default "defer_if_permit"
       action, the Postfix SMTP server continues to look for opportunities to
       reject mail, and defers the client request only if it would otherwise
       be accepted.

       For finer control, see: unverified_recipient_tempfail_action,
       unverified_sender_tempfail_action, unknown_address_tempfail_action, and
       unknown_helo_hostname_tempfail_action.

       This feature is available in Postfix 2.6 and later.

relay_clientcerts (default: empty)
       List of tables with remote SMTP client-certificate fingerprints or
       public key fingerprints (Postfix 2.9 and later) for which the Postfix
       SMTP server will allow access with the permit_tls_clientcerts feature.
       The fingerprint digest algorithm is configurable via the
       smtpd_tls_fingerprint_digest parameter (hard-coded as md5 prior to
       Postfix version 2.5).

       The default algorithm is sha256 with Postfix >= 3.6 and the
       compatibility_level set to 3.6 or higher. With Postfix <= 3.5, the
       default algorithm is md5.  The best-practice algorithm is now sha256.
       Recent advances in hash function cryptanalysis have led to md5 and sha1
       being deprecated in favor of sha256.  However, as long as there are no
       known "second pre-image" attacks against the older algorithms, their
       use in this context, though not recommended, is still likely safe.

       Postfix lookup tables are in the form of (key, value) pairs.  Since we
       only need the key, the value can be chosen freely, e.g.  the name of
       the user or host: D7:04:2F:A7:0B:8C:A5:21:FA:31:77:E1:41:8A:EE:80
       lutzpc.at.home

       Example:

       relay_clientcerts = hash:/etc/postfix/relay_clientcerts

       For more fine-grained control, use check_ccert_access to select an
       appropriate access(5) policy for each client.  See
       RESTRICTION_CLASS_README.

       This feature is available with Postfix version 2.2.

relay_destination_concurrency_limit (default:
       $default_destination_concurrency_limit)
       The maximal number of parallel deliveries to the same destination via
       the relay message delivery transport. This limit is enforced by the
       queue manager. The message delivery transport name is the first field
       in the entry in the master.cf file.

       This feature is available in Postfix 2.0 and later.

relay_destination_recipient_limit (default:
       $default_destination_recipient_limit)
       The maximal number of recipients per message for the relay message
       delivery transport. This limit is enforced by the queue manager. The
       message delivery transport name is the first field in the entry in the
       master.cf file.

       Setting this parameter to a value of 1 changes the meaning of
       relay_destination_concurrency_limit from concurrency per domain into
       concurrency per recipient.

       This feature is available in Postfix 2.0 and later.

relay_domains (default: Postfix >= 3.0: empty, Postfix < 3.0: $mydestination)
       What destination domains (and subdomains thereof) this system will
       relay mail to. For details about how the relay_domains value is used,
       see the description of the permit_auth_destination and
       reject_unauth_destination SMTP recipient restrictions.

       Domains that match $relay_domains are delivered with the
       $relay_transport mail delivery transport. The SMTP server validates
       recipient addresses with $relay_recipient_maps and rejects non-existent
       recipients. See also the relay domains address class in the
       ADDRESS_CLASS_README file.

       Note: Postfix will not automatically forward mail for domains that list
       this system as their primary or backup MX host. See the
       permit_mx_backup restriction in the postconf(5) manual page.

       Specify a list of host or domain names, "/file/name" patterns or
       "type:table" lookup tables, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace. A
       "/file/name" pattern is replaced by its contents; a "type:table" lookup
       table is matched when a (parent) domain appears as lookup key. Specify
       "!pattern" to exclude a domain from the list. The form "!/file/name" is
       supported only in Postfix version 2.4 and later.

       Pattern matching of domain names is controlled by the presence or
       absence of "relay_domains" in the parent_domain_matches_subdomains
       parameter value.

relay_domains_reject_code (default: 554)
       The numerical Postfix SMTP server response code when a client request
       is rejected by the reject_unauth_destination recipient restriction.

       Do not change this unless you have a complete understanding of RFC
       5321.

relay_recipient_maps (default: empty)
       Optional lookup tables with all valid addresses in the domains that
       match $relay_domains. Specify @domain as a wild-card for domains that
       have no valid recipient list, and become a source of backscatter mail:
       Postfix accepts spam for non-existent recipients and then floods
       innocent people with undeliverable mail.  Technically, tables listed
       with $relay_recipient_maps are used as lists: Postfix needs to know
       only if a lookup string is found or not, but it does not use the result
       from the table lookup.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       If this parameter is non-empty, then the Postfix SMTP server will
       reject mail to unknown relay users. This feature is off by default.

       See also the relay domains address class in the ADDRESS_CLASS_README
       file.

       Example:

       relay_recipient_maps = hash:/etc/postfix/relay_recipients

       This feature is available in Postfix 2.0 and later.

relay_transport (default: relay)
       The default mail delivery transport and next-hop destination for remote
       delivery to domains listed with $relay_domains. In order of decreasing
       precedence, the nexthop destination is taken from $relay_transport,
       $sender_dependent_relayhost_maps, $relayhost, or from the recipient
       domain. This information can be overruled with the transport(5) table.

       Specify a string of the form transport:nexthop, where transport is the
       name of a mail delivery transport defined in master.cf.  The :nexthop
       destination is optional; its syntax is documented in the manual page of
       the corresponding delivery agent.

       See also the relay domains address class in the ADDRESS_CLASS_README
       file.

       This feature is available in Postfix 2.0 and later.

relayhost (default: empty)
       The next-hop destination(s) for non-local mail; overrides non-local
       domains in recipient addresses. This information is overruled with
       relay_transport, sender_dependent_default_transport_maps,
       default_transport, sender_dependent_relayhost_maps and with the
       transport(5) table.

       On an intranet, specify the organizational domain name. If your
       internal DNS uses no MX records, specify the name of the intranet
       gateway host instead.

       In the case of SMTP or LMTP delivery, specify one or more destinations
       in the form of a domain name, hostname, hostname:port, [hostname]:port,
       [hostaddress] or [hostaddress]:port, separated by comma or whitespace.
       The form [hostname] turns off MX lookups. Multiple destinations are
       supported in Postfix 3.5 and later.

       If you're connected via UUCP, see the UUCP_README file for useful
       information.

       Examples:

       relayhost = $mydomain
       relayhost = [gateway.example.com]
       relayhost = mail1.example:587, mail2.example:587
       relayhost = [an.ip.add.ress]

relocated_maps (default: empty)
       Optional lookup tables with new contact information for users or
       domains that no longer exist.  The table format and lookups are
       documented in relocated(5).

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       If you use this feature, run "postmap /etc/postfix/relocated" to build
       the necessary DBM or DB file after change, then "postfix reload" to
       make the changes visible.

       Examples:

       relocated_maps = dbm:/etc/postfix/relocated
       relocated_maps = hash:/etc/postfix/relocated

remote_header_rewrite_domain (default: empty)
       Don't rewrite message headers from remote clients at all when this
       parameter is empty; otherwise, rewrite message headers and append the
       specified domain name to incomplete addresses.  The
       local_header_rewrite_clients parameter controls what clients Postfix
       considers local.

       Examples:

       The safe setting: append "domain.invalid" to incomplete header
       addresses from remote SMTP clients, so that those addresses cannot be
       confused with local addresses.

           remote_header_rewrite_domain = domain.invalid

       The default, purist, setting: don't rewrite headers from remote clients
       at all.

           remote_header_rewrite_domain =

require_home_directory (default: no)
       Require that a local(8) recipient's home directory exists before mail
       delivery is attempted. By default this test is disabled.  It can be
       useful for environments that import home directories to the mail server
       (IMPORTING HOME DIRECTORIES IS NOT RECOMMENDED).

reset_owner_alias (default: no)
       Reset the local(8) delivery agent's idea of the owner-alias attribute,
       when delivering mail to a child alias that does not have its own owner
       alias.

       This feature is available in Postfix 2.8 and later. With older Postfix
       releases, the behavior is as if this parameter is set to "yes".

       As documented in aliases(5), when an alias name has a companion alias
       named owner-name, this will replace the envelope sender address, so
       that delivery errors will be reported to the owner alias instead of the
       sender. This configuration is recommended for mailing lists.

       A less known property of the owner alias is that it also forces the
       local(8) delivery agent to write local and remote addresses from alias
       expansion to a new queue file, instead of attempting to deliver mail to
       local addresses as soon as they come out of alias expansion.

       Writing local addresses from alias expansion to a new queue file allows
       for robust handling of temporary delivery errors: errors with one local
       member have no effect on deliveries to other members of the list.  On
       the other hand, delivery to local addresses as soon as they come out of
       alias expansion is fragile: a temporary error with one local address
       from alias expansion will cause the entire alias to be expanded
       repeatedly until the error goes away, or until the message expires in
       the queue.  In that case, a problem with one list member results in
       multiple message deliveries to other list members.

       The default behavior of Postfix 2.8 and later is to keep the
       owner-alias attribute of the parent alias, when delivering mail to a
       child alias that does not have its own owner alias. Then, local
       addresses from that child alias will be written to a new queue file,
       and a temporary error with one local address will not affect delivery
       to other mailing list members.

       Unfortunately, older Postfix releases reset the owner-alias attribute
       when delivering mail to a child alias that does not have its own owner
       alias. To be precise, this resets only the decision to create a new
       queue file, not the decision to override the envelope sender address.
       The local(8) delivery agent then attempts to deliver local addresses as
       soon as they come out of child alias expansion.  If delivery to any
       address from child alias expansion fails with a temporary error
       condition, the entire mailing list may be expanded repeatedly until the
       mail expires in the queue, resulting in multiple deliveries of the same
       message to mailing list members.

resolve_dequoted_address (default: yes)
       Resolve a recipient address safely instead of correctly, by looking
       inside quotes.

       By default, the Postfix address resolver does not quote the address
       localpart as per RFC 822, so that additional @ or % or !  operators
       remain visible. This behavior is safe but it is also technically
       incorrect.

       If you specify "resolve_dequoted_address = no", then the Postfix
       resolver will not know about additional @ etc. operators in the address
       localpart. This opens opportunities for obscure mail relay attacks with
       user@domain@domain addresses when Postfix provides backup MX service
       for Sendmail systems.

resolve_null_domain (default: no)
       Resolve an address that ends in the "@" null domain as if the local
       hostname were specified, instead of rejecting the address as invalid.

       This feature is available in Postfix 2.1 and later.  Earlier versions
       always resolve the null domain as the local hostname.

       The Postfix SMTP server uses this feature to reject mail from or to
       addresses that end in the "@" null domain, and from addresses that
       rewrite into a form that ends in the "@" null domain.

resolve_numeric_domain (default: no)
       Resolve "user@ipaddress" as "user@[ipaddress]", instead of rejecting
       the address as invalid.

       This feature is available in Postfix 2.3 and later.

respectful_logging (default: see 'postconf -d' output)
       Avoid logging that implies white is better than black. Instead use
       'allowlist', 'denylist', and variations of those words.

       This feature is available in Postfix 3.6 and later.

rewrite_service_name (default: rewrite)
       The name of the address rewriting service. This service rewrites
       addresses to standard form and resolves them to a (delivery method,
       next-hop host, recipient) triple.

       This feature is available in Postfix 2.0 and later.

sample_directory (default: /etc/postfix)
       The name of the directory with example Postfix configuration files.
       Starting with Postfix 2.1, these files have been replaced with the
       postconf(5) manual page.

send_cyrus_sasl_authzid (default: no)
       When authenticating to a remote SMTP or LMTP server with the default
       setting "no", send no SASL authoriZation ID (authzid); send only the
       SASL authentiCation ID (authcid) plus the authcid's password.

       The non-default setting "yes" enables the behavior of older Postfix
       versions.  These always send a SASL authzid that is equal to the SASL
       authcid, but this causes interoperability problems with some SMTP
       servers.

       This feature is available in Postfix 2.4.4 and later.

sender_based_routing (default: no)
       This parameter should not be used. It was replaced by
       sender_dependent_relayhost_maps in Postfix version 2.3.

sender_bcc_maps (default: empty)
       Optional BCC (blind carbon-copy) address lookup tables, indexed by
       sender address.  The BCC address (multiple results are not supported)
       is added when mail enters from outside of Postfix.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       The table search order is as follows:

       ⊕      Look up the "user+extension@domain.tld" address including the
              optional address extension.

       ⊕      Look up the "user@domain.tld" address without the optional
              address extension.

       ⊕      Look up the "user+extension" address local part when the sender
              domain equals $myorigin, $mydestination, $inet_interfaces or
              $proxy_interfaces.

       ⊕      Look up the "user" address local part when the sender domain
              equals $myorigin, $mydestination, $inet_interfaces or
              $proxy_interfaces.

       ⊕      Look up the "@domain.tld" part.

       Note: with Postfix 2.3 and later the BCC address is added as if it was
       specified with NOTIFY=NONE. The sender will not be notified when the
       BCC address is undeliverable, as long as all down-stream software
       implements RFC 3461.

       Note: with Postfix 2.2 and earlier the sender will be notified when the
       BCC address is undeliverable.

       Note: automatic BCC recipients are produced only for new mail.  To
       avoid mailer loops, automatic BCC recipients are not generated after
       Postfix forwards mail internally, or after Postfix generates mail
       itself.

       Example:

       sender_bcc_maps = hash:/etc/postfix/sender_bcc

       After a change, run "postmap /etc/postfix/sender_bcc".

       This feature is available in Postfix 2.1 and later.

sender_canonical_classes (default: envelope_sender, header_sender)
       What addresses are subject to sender_canonical_maps address mapping.
       By default, sender_canonical_maps address mapping is applied to
       envelope sender addresses, and to header sender addresses.

       Specify one or more of: envelope_sender, header_sender

       This feature is available in Postfix 2.2 and later.

sender_canonical_maps (default: empty)
       Optional address mapping lookup tables for envelope and header sender
       addresses.  The table format and lookups are documented in
       canonical(5).

       Example: you want to rewrite the SENDER address "user@ugly.domain" to
       "user@pretty.domain", while still being able to send mail to the
       RECIPIENT address "user@ugly.domain".

       Note: $sender_canonical_maps is processed before $canonical_maps.

       Example:

       sender_canonical_maps = hash:/etc/postfix/sender_canonical

sender_dependent_default_transport_maps (default: empty)
       A sender-dependent override for the global default_transport parameter
       setting. The tables are searched by the envelope sender address and
       @domain. A lookup result of DUNNO terminates the search without
       overriding the global default_transport parameter setting.  This
       information is overruled with the transport(5) table.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       Note: this overrides default_transport, not transport_maps, and
       therefore the expected syntax is that of default_transport, not the
       syntax of transport_maps.  Specifically, this does not support the
       transport_maps syntax for null transport, null nexthop, or null email
       addresses.

       For safety reasons, this feature does not allow $number substitutions
       in regular expression maps.

       This feature is available in Postfix 2.7 and later.

sender_dependent_relayhost_maps (default: empty)
       A sender-dependent override for the global relayhost parameter setting.
       The tables are searched by the envelope sender address and @domain. A
       lookup result of DUNNO terminates the search without overriding the
       global relayhost parameter setting (Postfix 2.6 and later). This
       information is overruled with relay_transport,
       sender_dependent_default_transport_maps, default_transport and with the
       transport(5) table.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       For safety reasons, this feature does not allow $number substitutions
       in regular expression maps.

       This feature is available in Postfix 2.3 and later.

sendmail_fix_line_endings (default: always)
       Controls how the Postfix sendmail command converts email message line
       endings from <CR><LF> into UNIX format (<LF>).

       always Always convert message lines ending in <CR><LF>. This setting is
              the default with Postfix 2.9 and later.

       strict Convert message lines ending in <CR><LF> only if the first input
              line ends in <CR><LF>. This setting is backwards-compatible with
              Postfix 2.8 and earlier.

       never  Never convert message lines ending in <CR><LF>. This setting
              exists for completeness only.

       This feature is available in Postfix 2.9 and later.

sendmail_path (default: see postconf -d output)
       A Sendmail compatibility feature that specifies the location of the
       Postfix sendmail(1) command. This command can be used to submit mail
       into the Postfix queue.

service_name (read-only)
       The master.cf service name of a Postfix daemon process. This can be
       used to distinguish the logging from different services that use the
       same program name.

       Example master.cf entries:

       # Distinguish inbound MTA logging from submission and smtps logging.
       smtp      inet  n       -       n       -       -       smtpd
       submission inet n       -       n       -       -       smtpd
           -o syslog_name=postfix/$service_name
       smtps     inet  n       -       n       -       -       smtpd
           -o syslog_name=postfix/$service_name

       # Distinguish outbound MTA logging from inbound relay logging.
       smtp      unix  -       -       n       -       -       smtp
       relay     unix  -       -       n       -       -       smtp
           -o syslog_name=postfix/$service_name

service_throttle_time (default: 60s)
       How long the Postfix master(8) waits before forking a server that
       appears to be malfunctioning.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

setgid_group (default: postdrop)
       The group ownership of set-gid Postfix commands and of group-writable
       Postfix directories. When this parameter value is changed you need to
       re-run "postfix set-permissions" (with Postfix version 2.0 and earlier:
       "/etc/postfix/post-install set-permissions".

shlib_directory (default: see 'postconf -d' output)
       The location of Postfix dynamically-linked libraries (libpostfix-*.so),
       and the default location of Postfix database plugins (postfix-*.so)
       that have a relative pathname in the dynamicmaps.cf file.  The
       shlib_directory parameter defaults to "no" when Postfix
       dynamically-linked libraries and database plugins are disabled at
       compile time, otherwise it typically defaults to /usr/lib/postfix or
       /usr/local/lib/postfix.

       Notes:

       ⊕      The directory specified with shlib_directory should contain only
              Postfix-related files. Postfix dynamically-linked libraries and
              database plugins should not be installed in a "public" system
              directory such as /usr/lib or /usr/local/lib. Linking Postfix
              dynamically-linked library files or database plugins into
              non-Postfix programs is not supported.  Postfix
              dynamically-linked libraries and database plugins implement a
              Postfix-internal API that changes without maintaining
              compatibility.

       ⊕      You can change the shlib_directory value after Postfix is built.
              However, you may have to run ldconfig or equivalent to prevent
              Postfix programs from failing because the libpostfix-*.so files
              are not found.  No ldconfig command is needed if you keep the
              libpostfix-*.so files in the compiled-in default
              $shlib_directory location.

       This feature is available in Postfix 3.0 and later.

show_user_unknown_table_name (default: yes)
       Display the name of the recipient table in the "User unknown"
       responses.  The extra detail makes troubleshooting easier but also
       reveals information that is nobody else's business.

       This feature is available in Postfix 2.0 and later.

showq_service_name (default: showq)
       The name of the showq(8) service. This service produces mail queue
       status reports.

       This feature is available in Postfix 2.0 and later.

smtp_address_preference (default: any)
       The address type ("ipv6", "ipv4" or "any") that the Postfix SMTP client
       will try first, when a destination has IPv6 and IPv4 addresses with
       equal MX preference. This feature has no effect unless the
       inet_protocols setting enables both IPv4 and IPv6.

       Postfix SMTP client address preference has evolved. With Postfix 2.8
       the default is "ipv6"; earlier implementations are hard-coded to prefer
       IPv6 over IPv4.

       Notes for mail delivery between sites that have both IPv4 and IPv6
       connectivity:

       ⊕      The setting "smtp_address_preference = ipv6" is unsafe.  It can
              fail to deliver mail when there is an outage that affects IPv6,
              while the destination is still reachable over IPv4.

       ⊕      The setting "smtp_address_preference = any" is safe. With this,
              mail will eventually be delivered even if there is an outage
              that affects IPv6 or IPv4, as long as it does not affect both.

       This feature is available in Postfix 2.8 and later.

smtp_address_verify_target (default: rcpt)
       In the context of email address verification, the SMTP protocol stage
       that determines whether an email address is deliverable.  Specify one
       of "rcpt" or "data".  The latter is needed with remote SMTP servers
       that reject recipients after the DATA command. Use transport_maps to
       apply this feature selectively:

           /etc/postfix/main.cf:
               transport_maps = hash:/etc/postfix/transport

           /etc/postfix/transport:
               smtp-domain-that-verifies-after-data    smtp-data-target:
               lmtp-domain-that-verifies-after-data    lmtp-data-target:

           /etc/postfix/master.cf:
               smtp-data-target    unix    -    -    n    -    -    smtp
                   -o smtp_address_verify_target=data
               lmtp-data-target    unix    -    -    n    -    -    lmtp
                   -o lmtp_address_verify_target=data

       Unselective use of the "data" target does no harm, but will result in
       unnecessary "lost connection after DATA" events at remote SMTP/LMTP
       servers.

       This feature is available in Postfix 3.0 and later.

smtp_always_send_ehlo (default: yes)
       Always send EHLO at the start of an SMTP session.

       With "smtp_always_send_ehlo = no", the Postfix SMTP client sends EHLO
       only when the word "ESMTP" appears in the server greeting banner
       (example: 220 spike.porcupine.org ESMTP Postfix).

smtp_balance_inet_protocols (default: yes)
       When a remote destination resolves to a combination of IPv4 and IPv6
       addresses, ensure that the Postfix SMTP client can try both address
       types before it runs into the smtp_mx_address_limit.

       This avoids an interoperability problem when a destination resolves to
       primarily IPv6 addresses, the smtp_address_limit feature eliminates
       most or all IPv4 addresses, and the destination is not reachable over
       IPv6.

       This feature is available in Postfix 3.3 and later.

smtp_bind_address (default: empty)
       An optional numerical network address that the Postfix SMTP client
       should bind to when making an IPv4 connection.

       This can be specified in the main.cf file for all SMTP clients, or it
       can be specified in the master.cf file for a specific client, for
       example:

           /etc/postfix/master.cf:
               smtp ... smtp -o smtp_bind_address=11.22.33.44

       See smtp_bind_address_enforce for how Postfix should handle errors
       (Postfix 3.7 and later).

       Note 1: when inet_interfaces specifies no more than one IPv4 address,
       and that address is a non-loopback address, it is automatically used as
       the smtp_bind_address.  This supports virtual IP hosting, but can be a
       problem on multi-homed firewalls. See the inet_interfaces documentation
       for more detail.

       Note 2: address information may be enclosed inside [], but this form is
       not required here.

smtp_bind_address6 (default: empty)
       An optional numerical network address that the Postfix SMTP client
       should bind to when making an IPv6 connection.

       This feature is available in Postfix 2.2 and later.

       This can be specified in the main.cf file for all SMTP clients, or it
       can be specified in the master.cf file for a specific client, for
       example:

           /etc/postfix/master.cf:
               smtp ... smtp -o smtp_bind_address6=1:2:3:4:5:6:7:8

       See smtp_bind_address_enforce for how Postfix should handle errors
       (Postfix 3.7 and later).

       Note 1: when inet_interfaces specifies no more than one IPv6 address,
       and that address is a non-loopback address, it is automatically used as
       the smtp_bind_address6.  This supports virtual IP hosting, but can be a
       problem on multi-homed firewalls. See the inet_interfaces documentation
       for more detail.

       Note 2: address information may be enclosed inside [], but this form is
       not recommended here.

smtp_bind_address_enforce (default: no)
       Defer delivery when the Postfix SMTP client cannot apply the
       smtp_bind_address or smtp_bind_address6 setting. By default, the
       Postfix SMTP client will continue delivery after logging a warning.

       This feature is available in Postfix 3.7 and later.

smtp_body_checks (default: empty)
       Restricted body_checks(5) tables for the Postfix SMTP client.  These
       tables are searched while mail is being delivered.  Actions that change
       the delivery time or destination are not available.

       This feature is available in Postfix 2.5 and later.

smtp_cname_overrides_servername (default: version dependent)
       When the remote SMTP servername is a DNS CNAME, replace the servername
       with the result from CNAME expansion for the purpose of logging, SASL
       password lookup, TLS policy decisions, or TLS certificate verification.
       The value "no" hardens Postfix smtp_tls_per_site hostname-based
       policies against false hostname information in DNS CNAME records, and
       makes SASL password file lookups more predictable. This is the default
       setting as of Postfix 2.3.

       When DNS CNAME records are validated with secure DNS lookups
       (smtp_dns_support_level = dnssec), they are always allowed to override
       the above servername (Postfix 2.11 and later).

       This feature is available in Postfix 2.2.9 and later.

smtp_connect_timeout (default: 30s)
       The Postfix SMTP client time limit for completing a TCP connection, or
       zero (use the operating system built-in time limit).

       When no connection can be made within the deadline, the Postfix SMTP
       client tries the next address on the mail exchanger list. Specify 0 to
       disable the time limit (i.e. use whatever timeout is implemented by the
       operating system).

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtp_connection_cache_destinations (default: empty)
       Permanently enable SMTP connection caching for the specified
       destinations.  With SMTP connection caching, a connection is not closed
       immediately after completion of a mail transaction.  Instead, the
       connection is kept open for up to $smtp_connection_cache_time_limit
       seconds.  This allows connections to be reused for other deliveries,
       and can improve mail delivery performance.

       Specify a comma or white space separated list of destinations or
       pseudo-destinations:

       ⊕      if mail is sent without a relay host: a domain name (the
              right-hand side of an email address, without the [] around a
              numeric IP address),

       ⊕      if mail is sent via a relay host: a relay host name (without []
              or non-default TCP port), as specified in main.cf or in the
              transport map,

       ⊕      if mail is sent via a UNIX-domain socket: a pathname (without
              the unix: prefix),

       ⊕      a /file/name with domain names and/or relay host names as
              defined above,

       ⊕      a "type:table" with domain names and/or relay host names on the
              left-hand side.  The right-hand side result from "type:table"
              lookups is ignored.

       This feature is available in Postfix 2.2 and later.

smtp_connection_cache_on_demand (default: yes)
       Temporarily enable SMTP connection caching while a destination has a
       high volume of mail in the active queue.  With SMTP connection caching,
       a connection is not closed immediately after completion of a mail
       transaction.  Instead, the connection is kept open for up to
       $smtp_connection_cache_time_limit seconds.  This allows connections to
       be reused for other deliveries, and can improve mail delivery
       performance.

       This feature is available in Postfix 2.2 and later.

smtp_connection_cache_time_limit (default: 2s)
       When SMTP connection caching is enabled, the amount of time that an
       unused SMTP client socket is kept open before it is closed.  Do not
       specify larger values without permission from the remote sites.

       This feature is available in Postfix 2.2 and later.

smtp_connection_reuse_count_limit (default: 0)
       When SMTP connection caching is enabled, the number of times that an
       SMTP session may be reused before it is closed, or zero (no limit).
       With a reuse count limit of N, a connection is used up to N+1 times.

       NOTE: This feature is unsafe. When a high-volume destination has
       multiple inbound MTAs, then the slowest inbound MTA will attract the
       most connections to that destination.  This limitation does not exist
       with the smtp_connection_reuse_time_limit feature.

       This feature is available in Postfix 2.11.

smtp_connection_reuse_time_limit (default: 300s)
       The amount of time during which Postfix will use an SMTP connection
       repeatedly.  The timer starts when the connection is initiated (i.e. it
       includes the connect, greeting and helo latency, in addition to the
       latencies of subsequent mail delivery transactions).

       This feature addresses a performance stability problem with remote SMTP
       servers. This problem is not specific to Postfix: it can happen when
       any MTA sends large amounts of SMTP email to a site that has multiple
       MX hosts.

       The problem starts when one of a set of MX hosts becomes slower than
       the rest.  Even though SMTP clients connect to fast and slow MX hosts
       with equal probability, the slow MX host ends up with more simultaneous
       inbound connections than the faster MX hosts, because the slow MX host
       needs more time to serve each client request.

       The slow MX host becomes a connection attractor.  If one MX host
       becomes N times slower than the rest, it dominates mail delivery
       latency unless there are more than N fast MX hosts to counter the
       effect. And if the number of MX hosts is smaller than N, the mail
       delivery latency becomes effectively that of the slowest MX host
       divided by the total number of MX hosts.

       The solution uses connection caching in a way that differs from Postfix
       version 2.2.  By limiting the amount of time during which a connection
       can be used repeatedly (instead of limiting the number of deliveries
       over that connection), Postfix not only restores fairness in the
       distribution of simultaneous connections across a set of MX hosts, it
       also favors deliveries over connections that perform well, which is
       exactly what we want.

       The default reuse time limit, 300s, is comparable to the various smtp
       transaction timeouts which are fair estimates of maximum excess latency
       for a slow delivery.  Note that hosts may accept thousands of messages
       over a single connection within the default connection reuse time
       limit. This number is much larger than the default Postfix version 2.2
       limit of 10 messages per cached connection. It may prove necessary to
       lower the limit to avoid interoperability issues with MTAs that exhibit
       bugs when many messages are delivered via a single connection.  A lower
       reuse time limit risks losing the benefit of connection reuse when the
       average connection and mail delivery latency exceeds the reuse time
       limit.

       This feature is available in Postfix 2.3 and later.

smtp_data_done_timeout (default: 600s)
       The Postfix SMTP client time limit for sending the SMTP ".", and for
       receiving the remote SMTP server response.

       When no response is received within the deadline, a warning is logged
       that the mail may be delivered multiple times.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtp_data_init_timeout (default: 120s)
       The Postfix SMTP client time limit for sending the SMTP DATA command,
       and for receiving the remote SMTP server response.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
       The default time unit is s (seconds).

smtp_data_xfer_timeout (default: 180s)
       The Postfix SMTP client time limit for sending the SMTP message
       content.  When the connection makes no progress for more than
       $smtp_data_xfer_timeout seconds the Postfix SMTP client terminates the
       transfer.

       Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
       The default time unit is s (seconds).

smtp_defer_if_no_mx_address_found (default: no)
       Defer mail delivery when no MX record resolves to an IP address.

       The default (no) is to return the mail as undeliverable. With older
       Postfix versions the default was to keep trying to deliver the mail
       until someone fixed the MX record or until the mail was too old.

       Note: the Postfix SMTP client always ignores MX records with equal or
       worse preference than the local MTA itself.

       This feature is available in Postfix 2.1 and later.

smtp_delivery_status_filter (default: $default_delivery_status_filter)
       Optional filter for the smtp(8) delivery agent to change the delivery
       status code or explanatory text of successful or unsuccessful
       deliveries.  See default_delivery_status_filter for details.

       NOTE: This feature modifies Postfix SMTP client error or non-error
       messages that may or may not be derived from remote SMTP server
       responses.  In contrast, the smtp_reply_filter feature modifies remote
       SMTP server responses only.

smtp_destination_concurrency_limit (default:
       $default_destination_concurrency_limit)
       The maximal number of parallel deliveries to the same destination via
       the smtp message delivery transport. This limit is enforced by the
       queue manager. The message delivery transport name is the first field
       in the entry in the master.cf file.

smtp_destination_recipient_limit (default:
       $default_destination_recipient_limit)
       The maximal number of recipients per message for the smtp message
       delivery transport. This limit is enforced by the queue manager. The
       message delivery transport name is the first field in the entry in the
       master.cf file.

       Setting this parameter to a value of 1 changes the meaning of
       smtp_destination_concurrency_limit from concurrency per domain into
       concurrency per recipient.

smtp_discard_ehlo_keyword_address_maps (default: empty)
       Lookup tables, indexed by the remote SMTP server address, with case
       insensitive lists of EHLO keywords (pipelining, starttls, auth, etc.)
       that the Postfix SMTP client will ignore in the EHLO response from a
       remote SMTP server. See smtp_discard_ehlo_keywords for details. The
       table is not indexed by hostname for consistency with
       smtpd_discard_ehlo_keyword_address_maps.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       This feature is available in Postfix 2.2 and later.

smtp_discard_ehlo_keywords (default: empty)
       A case insensitive list of EHLO keywords (pipelining, starttls, auth,
       etc.) that the Postfix SMTP client will ignore in the EHLO response
       from a remote SMTP server.

       This feature is available in Postfix 2.2 and later.

       Notes:

       ⊕      Specify the silent-discard pseudo keyword to prevent this action
              from being logged.

       ⊕      Use the smtp_discard_ehlo_keyword_address_maps feature to
              discard EHLO keywords selectively.

smtp_dns_reply_filter (default: empty)
       Optional filter for Postfix SMTP client DNS lookup results.  Specify
       zero or more lookup tables.  The lookup tables are searched in the
       given order for a match with the DNS lookup result, converted to the
       following form:

           name ttl class type preference value

       The class field is always "IN", the preference field exists only for MX
       records, the names of hosts, domains, etc.  end in ".", and those names
       are in ASCII form (xn--mumble form in the case of UTF8 names).

       When a match is found, the table lookup result specifies an action.  By
       default, the table query and the action name are case-insensitive.
       Currently, only the IGNORE action is implemented.

       Notes:

       ⊕      Postfix DNS reply filters have no effect on implicit DNS lookups
              through nsswitch.conf or equivalent mechanisms.

       ⊕      The Postfix SMTP/LMTP client uses smtp_dns_reply_filter and
              lmtp_dns_reply_filter only to discover a remote SMTP or LMTP
              service (record types MX, A, AAAA, and TLSA).  These lookups are
              also made to implement the features reject_unverified_sender and
              reject_unverified_recipient.

       ⊕      The Postfix SMTP/LMTP client defers mail delivery when a filter
              removes all lookup results from a successful query.

       ⊕      Postfix SMTP server uses smtpd_dns_reply_filter only to look up
              MX, A, AAAA, and TXT records to implement the features
              reject_unknown_helo_hostname, reject_unknown_sender_domain,
              reject_unknown_recipient_domain, reject_rbl_*, and
              reject_rhsbl_*.

       ⊕      The Postfix SMTP server logs a warning or defers mail delivery
              when a filter removes all lookup results from a successful
              query.

       Example: ignore Google AAAA records in Postfix SMTP client DNS lookups,
       because Google sometimes hard-rejects mail from IPv6 clients with valid
       PTR etc. records.

       /etc/postfix/main.cf:
           smtp_dns_reply_filter = pcre:/etc/postfix/smtp_dns_reply_filter

       /etc/postfix/smtp_dns_reply_filter:
           # /domain ttl IN AAAA address/ action, all case-insensitive.
           # Note: the domain name ends in ".".
           /^\S+\.google\.com\.\s+\S+\s+\S+\s+AAAA\s+/ IGNORE

       This feature is available in Postfix 3.0 and later.

smtp_dns_resolver_options (default: empty)
       DNS Resolver options for the Postfix SMTP client.  Specify zero or more
       of the following options, separated by comma or whitespace.  Option
       names are case-sensitive. Some options refer to domain names that are
       specified in the file /etc/resolv.conf or equivalent.

       res_defnames
              Append the current domain name to single-component names (those
              that do not contain a "." character). This can produce incorrect
              results, and is the hard-coded behavior prior to Postfix 2.8.

       res_dnsrch
              Search for host names in the current domain and in parent
              domains. This can produce incorrect results and is therefore not
              recommended.

       This feature is available in Postfix 2.8 and later.

smtp_dns_support_level (default: empty)
       Level of DNS support in the Postfix SMTP client.  With
       "smtp_dns_support_level" left at its empty default value, the legacy
       "disable_dns_lookups" parameter controls whether DNS is enabled in the
       Postfix SMTP client, otherwise the legacy parameter is ignored.

       Specify one of the following:

       disabled
              Disable DNS lookups.  No MX lookups are performed and hostname
              to address lookups are unconditionally "native".  This setting
              is not appropriate for hosts that deliver mail to the public
              Internet.  Some obsolete how-to documents recommend disabling
              DNS lookups in some configurations with content_filters.  This
              is no longer required and strongly discouraged.

       enabled
              Enable DNS lookups.  Nexthop destination domains not enclosed in
              "[]" will be subject to MX lookups.  If "dns" and "native" are
              included in the "smtp_host_lookup" parameter value, DNS will be
              queried first to resolve MX-host A records, followed by "native"
              lookups if no answer is found in DNS.

       dnssec Enable DNSSEC lookups.  The "dnssec" setting differs from the
              "enabled" setting above in the following ways:

       ⊕      Any MX lookups will set RES_USE_DNSSEC and RES_USE_EDNS0 to
              request DNSSEC-validated responses. If the MX response is
              DNSSEC-validated the corresponding hostnames are considered
              validated.

       ⊕      The address lookups of validated hostnames are also validated,
              (provided of course "smtp_host_lookup" includes "dns", see
              below).

       ⊕      Temporary failures in DNSSEC-enabled hostname-to-address
              resolution block any "native" lookups.  Additional "native"
              lookups only happen when DNSSEC lookups hard-fail (NODATA or
              NXDOMAIN).

       The Postfix SMTP client considers non-MX "[nexthop]" and
       "[nexthop]:port" destinations equivalent to statically-validated MX
       records of the form "nexthop.  IN MX 0 nexthop."  Therefore, with
       "dnssec" support turned on, validated hostname-to-address lookups apply
       to the nexthop domain of any "[nexthop]" or "[nexthop]:port"
       destination.  This is also true for LMTP "inet:host" and
       "inet:host:port" destinations, as LMTP hostnames are never subject to
       MX lookups.

       The "dnssec" setting is recommended only if you plan to use the dane or
       dane-only TLS security level, otherwise enabling DNSSEC support in
       Postfix offers no additional security.  Postfix DNSSEC support relies
       on an upstream recursive nameserver that validates DNSSEC signatures.
       Such a DNS server will always filter out forged DNS responses, even
       when Postfix itself is not configured to use DNSSEC.

       When using Postfix DANE support the "smtp_host_lookup" parameter should
       include "dns", as DANE is not applicable to hosts resolved via "native"
       lookups.

       As mentioned above, Postfix is not a validating stub resolver; it
       relies on the system's configured DNSSEC-validating recursive
       nameserver to perform all DNSSEC validation.  Since this nameserver's
       DNSSEC-validated responses will be fully trusted, it is strongly
       recommended that the MTA host have a local DNSSEC-validating recursive
       caching nameserver listening on a loopback address, and be configured
       to use only this nameserver for all lookups.  Otherwise, Postfix may
       remain subject to man-in-the-middle attacks that forge responses from
       the recursive nameserver

       DNSSEC support requires a version of Postfix compiled against a
       reasonably-modern DNS resolver(3) library that implements the
       RES_USE_DNSSEC and RES_USE_EDNS0 resolver options.

       This feature is available in Postfix 2.11 and later.

smtp_enforce_tls (default: no)
       Enforcement mode: require that remote SMTP servers use TLS encryption,
       and never send mail in the clear.  This also requires that the remote
       SMTP server hostname matches the information in the remote server
       certificate, and that the remote SMTP server certificate was issued by
       a CA that is trusted by the Postfix SMTP client. If the certificate
       doesn't verify or the hostname doesn't match, delivery is deferred and
       mail stays in the queue.

       The server hostname is matched against all names provided as dNSNames
       in the SubjectAlternativeName.  If no dNSNames are specified, the
       CommonName is checked.  The behavior may be changed with the
       smtp_tls_enforce_peername option.

       This option is useful only if you are definitely sure that you will
       only connect to servers that support RFC 2487 _and_ that provide valid
       server certificates.  Typical use is for clients that send all their
       email to a dedicated mailhub.

       This feature is available in Postfix 2.2 and later. With Postfix 2.3
       and later use smtp_tls_security_level instead.

smtp_fallback_relay (default: $fallback_relay)
       Optional list of relay hosts for SMTP destinations that can't be found
       or that are unreachable. With Postfix 2.2 and earlier this parameter is
       called fallback_relay.

       By default, mail is returned to the sender when a destination is not
       found, and delivery is deferred when a destination is unreachable.

       With bulk email deliveries, it can be beneficial to run the fallback
       relay MTA on the same host, so that it can reuse the sender IP address.
       This speeds up deliveries that are delayed by IP-based reputation
       systems (greylist, etc.).

       The fallback relays must be SMTP destinations. Specify a domain, host,
       host:port, [host]:port, [address] or [address]:port; the form [host]
       turns off MX lookups.  If you specify multiple SMTP destinations,
       Postfix will try them in the specified order.

       To prevent mailer loops between MX hosts and fall-back hosts, Postfix
       version 2.2 and later will not use the fallback relays for destinations
       that it is MX host for (assuming DNS lookup is turned on).

smtp_generic_maps (default: empty)
       Optional lookup tables that perform address rewriting in the Postfix
       SMTP client, typically to transform a locally valid address into a
       globally valid address when sending mail across the Internet.  This is
       needed when the local machine does not have its own Internet domain
       name, but uses something like localdomain.local instead.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       The table format and lookups are documented in generic(5); examples are
       shown in the ADDRESS_REWRITING_README and STANDARD_CONFIGURATION_README
       documents.

       This feature is available in Postfix 2.2 and later.

smtp_header_checks (default: empty)
       Restricted header_checks(5) tables for the Postfix SMTP client.  These
       tables are searched while mail is being delivered.  Actions that change
       the delivery time or destination are not available.

       This feature is available in Postfix 2.5 and later.

smtp_helo_name (default: $myhostname)
       The hostname to send in the SMTP HELO or EHLO command.

       The default value is the machine hostname.  Specify a hostname or
       [ip.add.re.ss].

       This information can be specified in the main.cf file for all SMTP
       clients, or it can be specified in the master.cf file for a specific
       client, for example:

           /etc/postfix/master.cf:
               mysmtp ... smtp -o smtp_helo_name=foo.bar.com

       This feature is available in Postfix 2.0 and later.

smtp_helo_timeout (default: 300s)
       The Postfix SMTP client time limit for sending the HELO or EHLO
       command, and for receiving the initial remote SMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtp_host_lookup (default: dns)
       What mechanisms the Postfix SMTP client uses to look up a host's IP
       address.  This parameter is ignored when DNS lookups are disabled (see:
       disable_dns_lookups and smtp_dns_support_level).  The "dns" mechanism
       is always tried before "native" if both are listed.

       Specify one of the following:

       dns    Hosts can be found in the DNS (preferred).

       native Use the native naming service only (nsswitch.conf, or equivalent
              mechanism).

       dns, native
              Use the native service for hosts not found in the DNS.

       This feature is available in Postfix 2.1 and later.

smtp_line_length_limit (default: 998)
       The maximal length of message header and body lines that Postfix will
       send via SMTP. This limit does not include the <CR><LF> at the end of
       each line.  Longer lines are broken by inserting "<CR><LF><SPACE>", to
       minimize the damage to MIME formatted mail. Specify zero to disable
       this limit.

       The Postfix limit of 998 characters not including <CR><LF> is
       consistent with the SMTP limit of 1000 characters including <CR><LF>.
       The Postfix limit was 990 with Postfix 2.8 and earlier.

smtp_mail_timeout (default: 300s)
       The Postfix SMTP client time limit for sending the MAIL FROM command,
       and for receiving the remote SMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtp_mime_header_checks (default: empty)
       Restricted mime_header_checks(5) tables for the Postfix SMTP client.
       These tables are searched while mail is being delivered.  Actions that
       change the delivery time or destination are not available.

       This feature is available in Postfix 2.5 and later.

smtp_min_data_rate (default: 500)
       The minimum plaintext data transfer rate in bytes/second for DATA
       requests, when deadlines are enabled with smtp_per_request_deadline.
       After a write operation transfers N plaintext message bytes (possibly
       after TLS encryption), and after the DATA request deadline is
       decremented by the elapsed time of that write operation, the DATA
       request deadline is incremented by N/smtp_min_data_rate seconds.
       However, the deadline will never be incremented beyond the time limit
       specified with smtp_data_xfer_timeout.

       This feature is available in Postfix 3.7 and later.

smtp_mx_address_limit (default: 5)
       The maximal number of MX (mail exchanger) IP addresses that can result
       from Postfix SMTP client mail exchanger lookups, or zero (no limit).
       Prior to Postfix version 2.3, this limit was disabled by default.

       This feature is available in Postfix 2.1 and later.

smtp_mx_session_limit (default: 2)
       The maximal number of SMTP sessions per delivery request before the
       Postfix SMTP client gives up or delivers to a fall-back relay host, or
       zero (no limit). This restriction ignores sessions that fail to
       complete the SMTP initial handshake (Postfix version 2.2 and earlier)
       or that fail to complete the EHLO and TLS handshake (Postfix version
       2.3 and later).

       This feature is available in Postfix 2.1 and later.

smtp_nested_header_checks (default: empty)
       Restricted nested_header_checks(5) tables for the Postfix SMTP client.
       These tables are searched while mail is being delivered.  Actions that
       change the delivery time or destination are not available.

       This feature is available in Postfix 2.5 and later.

smtp_never_send_ehlo (default: no)
       Never send EHLO at the start of an SMTP session. See also the
       smtp_always_send_ehlo parameter.

smtp_per_record_deadline (default: no)
       Change the behavior of the smtp_*_timeout time limits, from a time
       limit per read or write system call, to a time limit to send or receive
       a complete record (an SMTP command line, SMTP response line, SMTP
       message content line, or TLS protocol message).  This limits the impact
       from hostile peers that trickle data one byte at a time.

       Note: when per-record deadlines are enabled, a short timeout may cause
       problems with TLS over very slow network connections.  The reasons are
       that a TLS protocol message can be up to 16 kbytes long (with TLSv1),
       and that an entire TLS protocol message must be sent or received within
       the per-record deadline.

       This feature is available in Postfix 2.9-3.6. With older Postfix
       releases, the behavior is as if this parameter is set to "no". Postfix
       3.7 and later use smtp_per_request_deadline.

smtp_per_request_deadline (default: no)
       Change the behavior of the smtp_*_timeout time limits, from a time
       limit per plaintext or TLS read or write call, to a combined time limit
       for sending a complete SMTP request and for receiving a complete SMTP
       response. The deadline limits only the time spent waiting for plaintext
       or TLS read or write calls, not time spent elsewhere. The per-request
       deadline limits the impact from hostile peers that trickle data one
       byte at a time.

       See smtp_min_data_rate for how the per-request deadline is managed
       during the DATA phase.

       Note: when per-request deadlines are enabled, a short time limit may
       cause problems with TLS over very slow network connections. The reason
       is that a TLS protocol message can be up to 16 kbytes long (with
       TLSv1), and that an entire TLS protocol message must be transferred
       within the per-request deadline.

       This feature is available in Postfix 3.7 and later. A weaker feature,
       called smtp_per_record_deadline, is available with Postfix 2.9-3.6.

       This feature is available in Postfix 3.7 and later.

smtp_pix_workaround_delay_time (default: 10s)
       How long the Postfix SMTP client pauses before sending ".<CR><LF>" in
       order to work around the PIX firewall "<CR><LF>.<CR><LF>" bug.

       Choosing too short a time makes this workaround ineffective when
       sending large messages over slow network connections.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtp_pix_workaround_maps (default: empty)
       Lookup tables, indexed by the remote SMTP server address, with
       per-destination workarounds for CISCO PIX firewall bugs.  The table is
       not indexed by hostname for consistency with
       smtp_discard_ehlo_keyword_address_maps.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       This feature is available in Postfix 2.4 and later.

smtp_pix_workaround_threshold_time (default: 500s)
       How long a message must be queued before the Postfix SMTP client turns
       on the PIX firewall "<CR><LF>.<CR><LF>" bug workaround for delivery
       through firewalls with "smtp fixup" mode turned on.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       By default, the workaround is turned off for mail that is queued for
       less than 500 seconds. In other words, the workaround is normally
       turned off for the first delivery attempt.

       Specify 0 to enable the PIX firewall "<CR><LF>.<CR><LF>" bug workaround
       upon the first delivery attempt.

smtp_pix_workarounds (default: disable_esmtp, delay_dotcrlf)
       A list that specifies zero or more workarounds for CISCO PIX firewall
       bugs. These workarounds are implemented by the Postfix SMTP client.
       Workaround names are separated by comma or space, and are case
       insensitive.  This parameter setting can be overruled with
       per-destination smtp_pix_workaround_maps settings.

       delay_dotcrlf
              Insert a delay before sending ".<CR><LF>" after the end of the
              message content.  The delay is subject to the
              smtp_pix_workaround_delay_time and
              smtp_pix_workaround_threshold_time parameter settings.

       disable_esmtp
              Disable all extended SMTP commands: send HELO instead of EHLO.

       This feature is available in Postfix 2.4 and later. The default
       settings are backwards compatible with earlier Postfix versions.

smtp_quit_timeout (default: 300s)
       The Postfix SMTP client time limit for sending the QUIT command, and
       for receiving the remote SMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtp_quote_rfc821_envelope (default: yes)
       Quote addresses in Postfix SMTP client MAIL FROM and RCPT TO commands
       as required by RFC 5321. This includes putting quotes around an address
       localpart that ends in ".".

       The default is to comply with RFC 5321. If you have to send mail to a
       broken SMTP server, configure a special SMTP client in master.cf:

           /etc/postfix/master.cf:
               broken-smtp . . . smtp -o smtp_quote_rfc821_envelope=no

       and route mail for the destination in question to the "broken-smtp"
       message delivery with a transport(5) table.

       This feature is available in Postfix 2.1 and later.

smtp_randomize_addresses (default: yes)
       Randomize the order of equal-preference MX host addresses.  This is a
       performance feature of the Postfix SMTP client.

smtp_rcpt_timeout (default: 300s)
       The Postfix SMTP client time limit for sending the SMTP RCPT TO
       command, and for receiving the remote SMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtp_reply_filter (default: empty)
       A mechanism to transform replies from remote SMTP servers one line at a
       time.  This is a last-resort tool to work around server replies that
       break interoperability with the Postfix SMTP client.  Other uses
       involve fault injection to test Postfix's handling of invalid
       responses.

       Notes:

       ⊕      In the case of a multi-line reply, the Postfix SMTP client uses
              the final reply line's numerical SMTP reply code and enhanced
              status code.

       ⊕      The numerical SMTP reply code (XYZ) takes precedence over the
              enhanced status code (X.Y.Z).  When the enhanced status code
              initial digit differs from the SMTP reply code initial digit, or
              when no enhanced status code is present, the Postfix SMTP client
              uses a generic enhanced status code (X.0.0) instead.

       Specify the name of a "type:table" lookup table. The search string is a
       single SMTP reply line as received from the remote SMTP server, except
       that the trailing <CR><LF> are removed.  When the lookup succeeds, the
       result replaces the single SMTP reply line.

       Examples:

       /etc/postfix/main.cf:
           smtp_reply_filter = pcre:/etc/postfix/reply_filter

       /etc/postfix/reply_filter:
           # Transform garbage into "250-filler..." so that it looks like
           # one line from a multi-line reply. It does not matter what we
           # substitute here as long it has the right syntax.  The Postfix
           # SMTP client will use the final line's numerical SMTP reply
           # code and enhanced status code.
           !/^([2-5][0-9][0-9]($|[- ]))/ 250-filler for garbage

       This feature is available in Postfix 2.7.

smtp_rset_timeout (default: 20s)
       The Postfix SMTP client time limit for sending the RSET command, and
       for receiving the remote SMTP server response. The SMTP client sends
       RSET in order to finish a recipient address probe, or to verify that a
       cached session is still usable.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

smtp_sasl_auth_cache_name (default: empty)
       An optional table to prevent repeated SASL authentication failures with
       the same remote SMTP server hostname, username and password. Each table
       (key, value) pair contains a server name, a username and password, and
       the full server response. This information is stored when a remote SMTP
       server rejects an authentication attempt with a 535 reply code.  As
       long as the smtp_sasl_password_maps information does not change, and as
       long as the smtp_sasl_auth_cache_name information does not expire (see
       smtp_sasl_auth_cache_time) the Postfix SMTP client avoids SASL
       authentication attempts with the same server, username and password,
       and instead bounces or defers mail as controlled with the
       smtp_sasl_auth_soft_bounce configuration parameter.

       Use a per-destination delivery concurrency of 1 (for example,
       "smtp_destination_concurrency_limit = 1",
       "relay_destination_concurrency_limit = 1", etc.), otherwise multiple
       delivery agents may experience a login failure at the same time.

       The table must be accessed via the proxywrite service, i.e. the map
       name must start with "proxy:". The table should be stored under the
       directory specified with the data_directory parameter.

       This feature uses cryptographic hashing to protect plain-text
       passwords, and requires that Postfix is compiled with TLS support.

       Example:

       smtp_sasl_auth_cache_name = proxy:btree:/var/db/postfix/sasl_auth_cache

       This feature is available in Postfix 2.5 and later.

smtp_sasl_auth_cache_time (default: 90d)
       The maximal age of an smtp_sasl_auth_cache_name entry before it is
       removed.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is d (days).

       This feature is available in Postfix 2.5 and later.

smtp_sasl_auth_enable (default: no)
       Enable SASL authentication in the Postfix SMTP client.  By default, the
       Postfix SMTP client uses no authentication.

       Example:

       smtp_sasl_auth_enable = yes

smtp_sasl_auth_soft_bounce (default: yes)
       When a remote SMTP server rejects a SASL authentication request with a
       535 reply code, defer mail delivery instead of returning mail as
       undeliverable. The latter behavior was hard-coded prior to Postfix
       version 2.5.

       Note: the setting "yes" overrides the global soft_bounce parameter, but
       the setting "no" does not.

       Example:

       # Default as of Postfix 2.5
       smtp_sasl_auth_soft_bounce = yes
       # The old hard-coded default
       smtp_sasl_auth_soft_bounce = no

       This feature is available in Postfix 2.5 and later.

smtp_sasl_mechanism_filter (default: empty)
       If non-empty, a Postfix SMTP client filter for the remote SMTP server's
       list of offered SASL mechanisms.  Different client and server
       implementations may support different mechanism lists; by default, the
       Postfix SMTP client will use the intersection of the two.
       smtp_sasl_mechanism_filter specifies an optional third mechanism list
       to intersect with.

       Specify mechanism names, "/file/name" patterns or "type:table" lookup
       tables. The right-hand side result from "type:table" lookups is
       ignored. Specify "!pattern" to exclude a mechanism name from the list.
       The form "!/file/name" is supported only in Postfix version 2.4 and
       later.

       This feature is available in Postfix 2.2 and later.

       Examples:

       smtp_sasl_mechanism_filter = plain, login
       smtp_sasl_mechanism_filter = /etc/postfix/smtp_mechs
       smtp_sasl_mechanism_filter = !gssapi, !login, static:rest

smtp_sasl_password_maps (default: empty)
       Optional Postfix SMTP client lookup tables with one username:password
       entry per sender, remote hostname or next-hop domain. Per-sender lookup
       is done only when sender-dependent authentication is enabled.  If no
       username:password entry is found, then the Postfix SMTP client will not
       attempt to authenticate to the remote host.

       The Postfix SMTP client opens the lookup table before going to chroot
       jail, so you can leave the password file in /etc/postfix.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

smtp_sasl_path (default: empty)
       Implementation-specific information that the Postfix SMTP client passes
       through to the SASL plug-in implementation that is selected with
       smtp_sasl_type.  Typically this specifies the name of a configuration
       file or rendezvous point.

       This feature is available in Postfix 2.3 and later.

smtp_sasl_security_options (default: noplaintext, noanonymous)
       Postfix SMTP client SASL security options; as of Postfix 2.3 the list
       of available features depends on the SASL client implementation that is
       selected with smtp_sasl_type.

       The following security features are defined for the cyrus client SASL
       implementation:

       Specify zero or more of the following:

       noplaintext
              Disallow methods that use plaintext passwords.

       noactive
              Disallow methods subject to active (non-dictionary) attack.

       nodictionary
              Disallow methods subject to passive (dictionary) attack.

       noanonymous
              Disallow methods that allow anonymous authentication.

       mutual_auth
              Only allow methods that provide mutual authentication (not
              available with SASL version 1).

       Example:

       smtp_sasl_security_options = noplaintext

smtp_sasl_tls_security_options (default: $smtp_sasl_security_options)
       The SASL authentication security options that the Postfix SMTP client
       uses for TLS encrypted SMTP sessions.

       This feature is available in Postfix 2.2 and later.

smtp_sasl_tls_verified_security_options (default:
       $smtp_sasl_tls_security_options)
       The SASL authentication security options that the Postfix SMTP client
       uses for TLS encrypted SMTP sessions with a verified server
       certificate.

       When mail is sent to the public MX host for the recipient's domain,
       server certificates are by default optional, and delivery proceeds even
       if certificate verification fails. For delivery via a submission
       service that requires SASL authentication, it may be appropriate to
       send plaintext passwords only when the connection to the server is
       strongly encrypted and the server identity is verified.

       The smtp_sasl_tls_verified_security_options parameter makes it possible
       to only enable plaintext mechanisms when a secure connection to the
       server is available. Submission servers subject to this policy must
       either have verifiable certificates or offer suitable non-plaintext
       SASL mechanisms.

       This feature is available in Postfix 2.6 and later.

smtp_sasl_type (default: cyrus)
       The SASL plug-in type that the Postfix SMTP client should use for
       authentication.  The available types are listed with the "postconf -A"
       command.

       This feature is available in Postfix 2.3 and later.

smtp_send_dummy_mail_auth (default: no)
       Whether or not to append the "AUTH=<>" option to the MAIL FROM command
       in SASL-authenticated SMTP sessions. The default is not to send this,
       to avoid problems with broken remote SMTP servers.  Before Postfix 2.9
       the behavior is as if "smtp_send_dummy_mail_auth = yes".

       This feature is available in Postfix 2.9 and later.

smtp_send_xforward_command (default: no)
       Send the non-standard XFORWARD command when the Postfix SMTP server
       EHLO response announces XFORWARD support.

       This allows a Postfix SMTP delivery agent, used for injecting mail into
       a content filter, to forward the name, address, protocol and HELO name
       of the original client to the content filter and downstream queuing
       SMTP server. This can produce more useful logging than
       localhost[127.0.0.1] etc.

       This feature is available in Postfix 2.1 and later.

smtp_sender_dependent_authentication (default: no)
       Enable sender-dependent authentication in the Postfix SMTP client; this
       is available only with SASL authentication, and disables SMTP
       connection caching to ensure that mail from different senders will use
       the appropriate credentials.

       This feature is available in Postfix 2.3 and later.

smtp_skip_4xx_greeting (default: yes)
       Skip SMTP servers that greet with a 4XX status code (go away, try again
       later).

       By default, the Postfix SMTP client moves on the next mail exchanger.
       Specify "smtp_skip_4xx_greeting = no" if Postfix should defer delivery
       immediately.

       This feature is available in Postfix 2.0 and earlier.  Later Postfix
       versions always skip remote SMTP servers that greet with a 4XX status
       code.

smtp_skip_5xx_greeting (default: yes)
       Skip remote SMTP servers that greet with a 5XX status code.

       By default, the Postfix SMTP client moves on the next mail exchanger.
       Specify "smtp_skip_5xx_greeting = no" if Postfix should bounce the mail
       immediately. Caution: the latter behavior appears to contradict RFC
       2821.

smtp_skip_quit_response (default: yes)
       Do not wait for the response to the SMTP QUIT command.

smtp_starttls_timeout (default: 300s)
       Time limit for Postfix SMTP client write and read operations during TLS
       startup and shutdown handshake procedures.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.2 and later.

smtp_tcp_port (default: smtp)
       The default TCP port that the Postfix SMTP client connects to.  Specify
       a symbolic name (see services(5)) or a numeric port.

smtp_tls_CAfile (default: empty)
       A file containing CA certificates of root CAs trusted to sign either
       remote SMTP server certificates or intermediate CA certificates.  These
       are loaded into memory before the smtp(8) client enters the chroot
       jail. If the number of trusted roots is large, consider using
       smtp_tls_CApath instead, but note that the latter directory must be
       present in the chroot jail if the smtp(8) client is chrooted. This file
       may also be used to augment the client certificate trust chain, but it
       is best to include all the required certificates directly in
       $smtp_tls_cert_file (or, Postfix >= 3.4 $smtp_tls_chain_files).

       Specify "smtp_tls_CAfile = /path/to/system_CA_file" to use ONLY the
       system-supplied default Certification Authority certificates.

       Specify "tls_append_default_CA = no" to prevent Postfix from appending
       the system-supplied default CAs and trusting third-party certificates.

       Example:

       smtp_tls_CAfile = /etc/postfix/CAcert.pem

       This feature is available in Postfix 2.2 and later.

smtp_tls_CApath (default: empty)
       Directory with PEM format Certification Authority certificates that the
       Postfix SMTP client uses to verify a remote SMTP server certificate.
       Don't forget to create the necessary "hash" links with, for example,
       "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs".

       To use this option in chroot mode, this directory (or a copy) must be
       inside the chroot jail.

       Specify "smtp_tls_CApath = /path/to/system_CA_directory" to use ONLY
       the system-supplied default Certification Authority certificates.

       Specify "tls_append_default_CA = no" to prevent Postfix from appending
       the system-supplied default CAs and trusting third-party certificates.

       Example:

       smtp_tls_CApath = /etc/postfix/certs

       This feature is available in Postfix 2.2 and later.

smtp_tls_block_early_mail_reply (default: no)
       Try to detect a mail hijacking attack based on a TLS protocol
       vulnerability (CVE-2009-3555), where an attacker prepends malicious
       HELO, MAIL, RCPT, DATA commands to a Postfix SMTP client TLS session.
       The attack would succeed with non-Postfix SMTP servers that reply to
       the malicious HELO, MAIL, RCPT, DATA commands after negotiating the
       Postfix SMTP client TLS session.

       This feature is available in Postfix 2.7.

smtp_tls_cert_file (default: empty)
       File with the Postfix SMTP client RSA certificate in PEM format.  This
       file may also contain the Postfix SMTP client private RSA key, and
       these may be the same as the Postfix SMTP server RSA certificate and
       key file.  With Postfix >= 3.4 the preferred way to configure client
       keys and certificates is via the "smtp_tls_chain_files" parameter.

       Do not configure client certificates unless you must present client TLS
       certificates to one or more servers. Client certificates are not
       usually needed, and can cause problems in configurations that work well
       without them. The recommended setting is to let the defaults stand:

           smtp_tls_cert_file =
           smtp_tls_key_file =
           smtp_tls_eccert_file =
           smtp_tls_eckey_file =
           # Obsolete DSA parameters
           smtp_tls_dcert_file =
           smtp_tls_dkey_file =
           # Postfix >= 3.4 interface
           smtp_tls_chain_files =

       The best way to use the default settings is to comment out the above
       parameters in main.cf if present.

       To enable remote SMTP servers to verify the Postfix SMTP client
       certificate, the issuing CA certificates must be made available to the
       server. You should include the required certificates in the client
       certificate file, the client certificate first, then the issuing CA(s)
       (bottom-up order).

       Example: the certificate for "client.example.com" was issued by
       "intermediate CA" which itself has a certificate issued by "root CA".
       As the "root" super-user create the client.pem file with:

           # umask 077
           # cat client_key.pem client_cert.pem intermediate_CA.pem > chain.pem

       If you also want to verify remote SMTP server certificates issued by
       these CAs, you can add the CA certificates to the smtp_tls_CAfile, in
       which case it is not necessary to have them in the smtp_tls_cert_file,
       smtp_tls_dcert_file (obsolete) or smtp_tls_eccert_file.

       A certificate supplied here must be usable as an SSL client certificate
       and hence pass the "openssl verify -purpose sslclient ..." test.

       Example:

       smtp_tls_cert_file = /etc/postfix/chain.pem

       This feature is available in Postfix 2.2 and later.

smtp_tls_chain_files (default: empty)
       List of one or more PEM files, each holding one or more private keys
       directly followed by a corresponding certificate chain.  The file names
       are separated by commas and/or whitespace.  This parameter obsoletes
       the legacy algorithm-specific key and certificate file settings.  When
       this parameter is non-empty, the legacy parameters are ignored, and a
       warning is logged if any are also non-empty.

       With the proliferation of multiple private key algorithms-which, as of
       OpenSSL 1.1.1, include DSA (obsolete), RSA, ECDSA, Ed25519 and Ed448-it
       is increasingly impractical to use separate parameters to configure the
       key and certificate chain for each algorithm.  Therefore, Postfix now
       supports storing multiple keys and corresponding certificate chains in
       a single file or in a set of files.

       Each key must appear immediately before the corresponding certificate,
       optionally followed by additional issuer certificates that complete the
       certificate chain for that key.  When multiple files are specified,
       they are equivalent to a single file that is concatenated from those
       files in the given order.  Thus, while a key must always precede its
       certificate and issuer chain, it can be in a separate file, so long as
       that file is listed immediately before the file that holds the
       corresponding certificate chain.  Once all the files are concatenated,
       the sequence of PEM objects must be: key1, cert1, [chain1], key2,
       cert2, [chain2], ..., keyN, certN, [chainN].

       Storing the private key in the same file as the corresponding
       certificate is more reliable.  With the key and certificate in separate
       files, there is a chance that during key rollover a Postfix process
       might load a private key and certificate from separate files that don't
       match.  Various operational errors may even result in a persistent
       broken configuration in which the certificate does not match the
       private key.

       The file or files must contain at most one key of each type.  If, for
       example, two or more RSA keys and corresponding chains are listed,
       depending on the version of OpenSSL either only the last one will be
       used or a configuration error may be detected.  Note that while
       "Ed25519" and "Ed448" are considered separate algorithms, the various
       ECDSA curves (typically one of prime256v1, secp384r1 or secp521r1) are
       considered as different parameters of a single "ECDSA" algorithm, so it
       is not presently possible to configure keys for more than one ECDSA
       curve.

       Example (separate files for each key and corresponding certificate
       chain):

           /etc/postfix/main.cf:
               smtp_tls_chain_files =
                   ${config_directory}/ed25519.pem,
                   ${config_directory}/ed448.pem,
                   ${config_directory}/rsa.pem

           /etc/postfix/ed25519.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----

           /etc/postfix/ed448.pem:
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----

           /etc/postfix/rsa.pem:
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       Example (all keys and certificates in a single file):

           /etc/postfix/main.cf:
               smtp_tls_chain_files = ${config_directory}/chains.pem

           /etc/postfix/chains.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       This feature is available in Postfix 3.4 and later.

smtp_tls_cipherlist (default: empty)
       Obsolete Postfix < 2.3 control for the Postfix SMTP client TLS cipher
       list. As this feature applies to all TLS security levels, it is easy to
       create interoperability problems by choosing a non-default cipher list.
       Do not use a non-default TLS cipher list on hosts that deliver email to
       the public Internet: you will be unable to send email to servers that
       only support the ciphers you exclude. Using a restricted cipher list
       may be more appropriate for an internal MTA, where one can exert some
       control over the TLS software and settings of the peer servers.

       Note: do not use "" quotes around the parameter value.

       This feature is available in Postfix version 2.2. It is not used with
       Postfix 2.3 and later; use smtp_tls_mandatory_ciphers instead.

smtp_tls_ciphers (default: medium)
       The minimum TLS cipher grade that the Postfix SMTP client will use with
       opportunistic TLS encryption. Cipher types listed in
       smtp_tls_exclude_ciphers are excluded from the base definition of the
       selected cipher grade.   The default value is "medium" for Postfix
       releases after the middle of 2015, "export" for older releases.

       When TLS is mandatory the cipher grade is chosen via the
       smtp_tls_mandatory_ciphers configuration parameter, see there for
       syntax details. See smtp_tls_policy_maps for information on how to
       configure ciphers on a per-destination basis.

       This feature is available in Postfix 2.6 and later. With earlier
       Postfix releases only the smtp_tls_mandatory_ciphers parameter is
       implemented, and opportunistic TLS always uses "export" or better (i.e.
       all) ciphers.

smtp_tls_connection_reuse (default: no)
       Try to make multiple deliveries per TLS-encrypted connection.  This
       uses the tlsproxy(8) service to encrypt an SMTP connection, uses the
       scache(8) service to save that connection, and relies on hints from the
       qmgr(8) daemon.

       See "Client-side TLS connection reuse" for background details.

       This feature is available in Postfix 3.4 and later.

smtp_tls_dane_insecure_mx_policy (default: see postconf -d output)
       The TLS policy for MX hosts with "secure" TLSA records when the nexthop
       destination security level is dane, but the MX record was found via an
       "insecure" MX lookup.  The choices are:

       may    The TLSA records will be ignored and TLS will be optional.  If
              the MX host does not appear to support STARTTLS, or the STARTTLS
              handshake fails, mail may be sent in the clear.

       encrypt
              The TLSA records will signal a requirement to use TLS.  While
              TLS encryption will be required, authentication will not be
              performed.

       dane   The TLSA records will be used just as with "secure" MX records.
              TLS encryption will be required, and, if at least one of the
              TLSA records is "usable", authentication will be required.  When
              authentication succeeds, it will be logged only as "Trusted",
              not "Verified", because the MX host name could have been forged.
              The default setting for Postfix >= 3.6 is "dane" with
              "smtp_tls_security_level = dane", otherwise "may". This behavior
              was backported to Postfix versions 3.5.9, 3.4.19, 3.3.16.
              3.2.21.  With earlier Postfix versions the default setting was
              always "dane".

       Though with "insecure" MX records an active attacker can compromise
       SMTP transport security by returning forged MX records, such attacks
       are "tamper-evident" since any forged MX hostnames will be recorded in
       the mail logs.  Attackers who place a high value on staying hidden may
       be deterred from forging MX records.

       This feature is available in Postfix 3.1 and later. The may policy is
       backwards-compatible with earlier Postfix versions.

smtp_tls_dcert_file (default: empty)
       File with the Postfix SMTP client DSA certificate in PEM format.  This
       file may also contain the Postfix SMTP client private DSA key.  The DSA
       algorithm is obsolete and should not be used.

       See the discussion under smtp_tls_cert_file for more details.

       Example:

       smtp_tls_dcert_file = /etc/postfix/client-dsa.pem

       This feature is available in Postfix 2.2 and later.

smtp_tls_dkey_file (default: $smtp_tls_dcert_file)
       File with the Postfix SMTP client DSA private key in PEM format.  This
       file may be combined with the Postfix SMTP client DSA certificate file
       specified with $smtp_tls_dcert_file. The DSA algorithm is obsolete and
       should not be used.

       The private key must be accessible without a pass-phrase, i.e. it must
       not be encrypted. File permissions should grant read-only access to the
       system superuser account ("root"), and no access to anyone else.

       This feature is available in Postfix 2.2 and later.

smtp_tls_eccert_file (default: empty)
       File with the Postfix SMTP client ECDSA certificate in PEM format.
       This file may also contain the Postfix SMTP client ECDSA private key.
       With Postfix >= 3.4 the preferred way to configure client keys and
       certificates is via the "smtp_tls_chain_files" parameter.

       See the discussion under smtp_tls_cert_file for more details.

       Example:

       smtp_tls_eccert_file = /etc/postfix/ecdsa-ccert.pem

       This feature is available in Postfix 2.6 and later, when Postfix is
       compiled and linked with OpenSSL 1.0.0 or later.

smtp_tls_eckey_file (default: $smtp_tls_eccert_file)
       File with the Postfix SMTP client ECDSA private key in PEM format.
       This file may be combined with the Postfix SMTP client ECDSA
       certificate file specified with $smtp_tls_eccert_file.  With Postfix >=
       3.4 the preferred way to configure client keys and certificates is via
       the "smtp_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must
       not be encrypted. File permissions should grant read-only access to the
       system superuser account ("root"), and no access to anyone else.

       This feature is available in Postfix 2.6 and later, when Postfix is
       compiled and linked with OpenSSL 1.0.0 or later.

smtp_tls_enforce_peername (default: yes)
       With mandatory TLS encryption, require that the remote SMTP server
       hostname matches the information in the remote SMTP server certificate.
       As of RFC 2487 the requirements for hostname checking for MTA clients
       are not specified.

       This option can be set to "no" to disable strict peer name checking.
       This setting has no effect on sessions that are controlled via the
       smtp_tls_per_site table.

       Disabling the hostname verification can make sense in a closed
       environment where special CAs are created.  If not used carefully, this
       option opens the danger of a "man-in-the-middle" attack (the CommonName
       of this attacker will be logged).

       This feature is available in Postfix 2.2 and later. With Postfix 2.3
       and later use smtp_tls_security_level instead.

smtp_tls_exclude_ciphers (default: empty)
       List of ciphers or cipher types to exclude from the Postfix SMTP client
       cipher list at all TLS security levels. This is not an OpenSSL
       cipherlist, it is a simple list separated by whitespace and/or commas.
       The elements are a single cipher, or one or more "+" separated cipher
       properties, in which case only ciphers matching all the properties are
       excluded.

       Examples (some of these will cause problems):

           smtp_tls_exclude_ciphers = aNULL
           smtp_tls_exclude_ciphers = MD5, DES
           smtp_tls_exclude_ciphers = DES+MD5
           smtp_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
           smtp_tls_exclude_ciphers = kEDH+aRSA

       The first setting disables anonymous ciphers. The next setting disables
       ciphers that use the MD5 digest algorithm or the (single) DES
       encryption algorithm. The next setting disables ciphers that use MD5
       and DES together.  The next setting disables the two ciphers
       "AES256-SHA" and "DES-CBC3-MD5". The last setting disables ciphers that
       use "EDH" key exchange with RSA authentication.

       This feature is available in Postfix 2.3 and later.

smtp_tls_fingerprint_cert_match (default: empty)
       List of acceptable remote SMTP server certificate fingerprints for the
       "fingerprint" TLS security level (smtp_tls_security_level =
       fingerprint). At this security level, Certification Authorities are not
       used, and certificate expiration times are ignored. Instead, server
       certificates are verified directly via their certificate fingerprint or
       public key fingerprint (Postfix 2.9 and later). The fingerprint is a
       message digest of the server certificate (or public key). The digest
       algorithm is selected via the smtp_tls_fingerprint_digest parameter.

       The colons between each pair of nibbles in the fingerprint value are
       optional (Postfix >= 3.6). These were required in earlier Postfix
       releases.

       When an smtp_tls_policy_maps table entry specifies the "fingerprint"
       security level, any "match" attributes in that entry specify the list
       of valid fingerprints for the corresponding destination. Multiple
       fingerprints can be combined with a "|" delimiter in a single match
       attribute, or multiple match attributes can be employed.

       Example: Certificate fingerprint verification with internal mailhub.
       Two matching fingerprints are listed. The relayhost may be multiple
       physical hosts behind a load-balancer, each with its own private/public
       key and self-signed certificate. Alternatively, a single relayhost may
       be in the process of switching from one set of private/public keys to
       another, and both keys are trusted just prior to the transition.

           relayhost = [mailhub.example.com]
           smtp_tls_security_level = fingerprint
           smtp_tls_fingerprint_digest = sha256
           smtp_tls_fingerprint_cert_match =
               cd:fc:d8:db:f8:c4:82:96:6c:...:28:71:e8:f5:8d:a5:0d:9b:d4:a6
               dd:5c:ef:f5:c3:bc:64:25:36:...:99:36:06:ce:40:ef:de:2e:ad:a4

       Example: Certificate fingerprint verification with selected
       destinations.  As in the example above, we show two matching
       fingerprints:

           /etc/postfix/main.cf:
               smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
               smtp_tls_fingerprint_digest = sha256

           /etc/postfix/tls_policy:
               example.com fingerprint
                   match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76
                   match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f

       This feature is available in Postfix 2.5 and later.

smtp_tls_fingerprint_digest (default: see postconf -d output)
       The message digest algorithm used to construct remote SMTP server
       certificate fingerprints. At the "fingerprint" TLS security level
       (smtp_tls_security_level = fingerprint), the server certificate is
       verified by directly matching its certificate fingerprint or its public
       key fingerprint (Postfix 2.9 and later). The fingerprint is the message
       digest of the server certificate (or its public key) using the selected
       algorithm. With a digest algorithm resistant to "second pre-image"
       attacks, it is not feasible to create a new public key and a matching
       certificate (or public/private key-pair) that has the same fingerprint.

       The default algorithm is sha256 with Postfix >= 3.6 and the
       compatibility_level set to 3.6 or higher. With Postfix <= 3.5, the
       default algorithm is md5.

       The best-practice algorithm is now sha256. Recent advances in hash
       function cryptanalysis have led to md5 and sha1 being deprecated in
       favor of sha256.  However, as long as there are no known "second
       pre-image" attacks against the older algorithms, their use in this
       context, though not recommended, is still likely safe.

       While additional digest algorithms are often available with OpenSSL's
       libcrypto, only those used by libssl in SSL cipher suites are available
       to Postfix.  You'll likely find support for md5, sha1, sha256 and
       sha512.

       To find the fingerprint of a specific certificate file, with a specific
       digest algorithm, run:

           $ openssl x509 -noout -fingerprint -digest -in certfile.pem

       The text to the right of the "=" sign is the desired fingerprint.  For
       example:

           $ openssl x509 -noout -fingerprint -sha256 -in cert.pem
           SHA256 Fingerprint=D4:6A:AB:19:24:...:BB:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A

       To extract the public key fingerprint from an X.509 certificate, you
       need to extract the public key from the certificate and compute the
       appropriate digest of its DER (ASN.1) encoding. With OpenSSL the
       "-pubkey" option of the "x509" command extracts the public key always
       in "PEM" format. We pipe the result to another OpenSSL command that
       converts the key to DER and then to the "dgst" command to compute the
       fingerprint.

       The actual command to transform the key to DER format depends on the
       version of OpenSSL used. As of OpenSSL 1.0.0, the "pkey" command
       supports all key types.

           # OpenSSL >= 1.0 with SHA-256 fingerprints.
           $ openssl x509 -in cert.pem -noout -pubkey |
               openssl pkey -pubin -outform DER |
               openssl dgst -sha256 -c
           (stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:...:fc:09:1a:61:98:b5:bc:7c:60:58

       The Postfix SMTP server and client log the peer (leaf) certificate
       fingerprint and the public key fingerprint when the TLS loglevel is 2
       or higher.

       This feature is available in Postfix 2.5 and later.

smtp_tls_force_insecure_host_tlsa_lookup (default: no)
       Lookup the associated DANE TLSA RRset even when a hostname is not an
       alias and its address records lie in an unsigned zone.  This is
       unlikely to ever yield DNSSEC validated results, since child zones of
       unsigned zones are also unsigned in the absence of DLV or locally
       configured non-root trust-anchors.  We anticipate that such mechanisms
       will not be used for just the "_tcp" subdomain of a host.  Suppressing
       the TLSA RRset lookup reduces latency and avoids potential
       interoperability problems with nameservers for unsigned zones that are
       not prepared to handle the new TLSA RRset.

       This feature is available in Postfix 2.11.

smtp_tls_key_file (default: $smtp_tls_cert_file)
       File with the Postfix SMTP client RSA private key in PEM format.  This
       file may be combined with the Postfix SMTP client RSA certificate file
       specified with $smtp_tls_cert_file.  With Postfix >= 3.4 the preferred
       way to configure client keys and certificates is via the
       "smtp_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must
       not be encrypted. File permissions should grant read-only access to the
       system superuser account ("root"), and no access to anyone else.

       Example:

       smtp_tls_key_file = $smtp_tls_cert_file

       This feature is available in Postfix 2.2 and later.

smtp_tls_loglevel (default: 0)
       Enable additional Postfix SMTP client logging of TLS activity.  Each
       logging level also includes the information that is logged at a lower
       logging level.


              0 Disable logging of TLS activity.


              1 Log only a summary message on TLS handshake completion - no
              logging of remote SMTP server certificate trust-chain
              verification errors if server certificate verification is not
              required.  With Postfix 2.8 and earlier, log the summary message
              and unconditionally log trust-chain verification errors.


              2 Also log levels during TLS negotiation.


              3 Also log the hexadecimal and ASCII dump of the TLS negotiation
              process.


              4 Also log the hexadecimal and ASCII dump of complete
              transmission after STARTTLS.

       Do not use "smtp_tls_loglevel = 2" or higher except in case of
       problems. Use of loglevel 4 is strongly discouraged.

       This feature is available in Postfix 2.2 and later.

smtp_tls_mandatory_ciphers (default: medium)
       The minimum TLS cipher grade that the Postfix SMTP client will use with
       mandatory TLS encryption.  The default value "medium" is suitable for
       most destinations with which you may want to enforce TLS, and is beyond
       the reach of today's cryptanalytic methods. See smtp_tls_policy_maps
       for information on how to configure ciphers on a per-destination basis.

       The following cipher grades are supported:

       export Enable "EXPORT" grade or better OpenSSL ciphers.  The underlying
              cipherlist is specified via the tls_export_cipherlist
              configuration parameter, which you are strongly encouraged not
              to change.  This choice is insecure and SHOULD NOT be used.

       low    Enable "LOW" grade or better OpenSSL ciphers.  The underlying
              cipherlist is specified via the tls_low_cipherlist configuration
              parameter, which you are strongly encouraged not to change.
              This choice is insecure and SHOULD NOT be used.

       medium Enable "MEDIUM" grade or better OpenSSL ciphers.  The underlying
              cipherlist is specified via the tls_medium_cipherlist
              configuration parameter, which you are strongly encouraged not
              to change.

       high   Enable only "HIGH" grade OpenSSL ciphers.  This setting may be
              appropriate when all mandatory TLS destinations (e.g. when all
              mail is routed to a suitably capable relayhost) support at least
              one "HIGH" grade cipher. The underlying cipherlist is specified
              via the tls_high_cipherlist configuration parameter, which you
              are strongly encouraged not to change.

       null   Enable only the "NULL" OpenSSL ciphers, these provide
              authentication without encryption.  This setting is only
              appropriate in the rare case that all servers are prepared to
              use NULL ciphers (not normally enabled in TLS servers). A
              plausible use-case is an LMTP server listening on a UNIX-domain
              socket that is configured to support "NULL" ciphers. The
              underlying cipherlist is specified via the tls_null_cipherlist
              configuration parameter, which you are strongly encouraged not
              to change.

       The underlying cipherlists for grades other than "null" include
       anonymous ciphers, but these are automatically filtered out if the
       Postfix SMTP client is configured to verify server certificates.  You
       are very unlikely to need to take any steps to exclude anonymous
       ciphers, they are excluded automatically as necessary.  If you must
       exclude anonymous ciphers at the "may" or "encrypt" security levels,
       when the Postfix SMTP client does not need or use peer certificates,
       set "smtp_tls_exclude_ciphers = aNULL". To exclude anonymous ciphers
       only when TLS is enforced, set "smtp_tls_mandatory_exclude_ciphers =
       aNULL".

       This feature is available in Postfix 2.3 and later.

smtp_tls_mandatory_exclude_ciphers (default: empty)
       Additional list of ciphers or cipher types to exclude from the Postfix
       SMTP client cipher list at mandatory TLS security levels. This list
       works in addition to the exclusions listed with
       smtp_tls_exclude_ciphers (see there for syntax details).

       Starting with Postfix 2.6, the mandatory cipher exclusions can be
       specified on a per-destination basis via the TLS policy "exclude"
       attribute. See smtp_tls_policy_maps for notes and examples.

       This feature is available in Postfix 2.3 and later.

smtp_tls_mandatory_protocols (default: see postconf -d output)
       TLS protocols that the Postfix SMTP client will use with mandatory TLS
       encryption.  In main.cf the values are separated by whitespace, commas
       or colons. In the policy table "protocols" attribute (see
       smtp_tls_policy_maps) the only valid separator is colon. An empty value
       means allow all protocols.

       The valid protocol names (see SSL_get_version(3)) are "SSLv2", "SSLv3",
       "TLSv1", "TLSv1.1", "TLSv1.2" and "TLSv1.3".  Starting with Postfix
       3.6, the default value is ">=TLSv1", which sets TLS 1.0 as the lowest
       supported TLS protocol version (see below).  Older releases use the "!"
       exclusion syntax, also described below.

       As of Postfix 3.6, the preferred way to limit the range of acceptable
       protocols is to set a lowest acceptable TLS protocol version and/or a
       highest acceptable TLS protocol version.  To set the lower bound
       include an element of the form: ">=version" where version is a either
       one of the TLS protocol names listed above, or a hexadecimal number
       corresponding to the desired TLS protocol version (0301 for TLS 1.0,
       0302 for TLS 1.1, etc.).  For the upper bound, use "<=version".  There
       must be no whitespace between the ">=" or "<=" symbols and the protocol
       name or number.

       Hexadecimal protocol numbers make it possible to specify protocol
       bounds for TLS versions that are known to OpenSSL, but might not be
       known to Postfix.  They cannot be used with the legacy exclusion
       syntax.  Leading "0" or "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent to
       "TLSv1".  Hexadecimal versions unknown to OpenSSL will fail to set the
       upper or lower bound, and a warning will be logged.  Hexadecimal
       versions should only be used when Postfix is linked with some future
       version of OpenSSL that supports TLS 1.4 or later, but Postfix does not
       yet support a symbolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtp_tls_mandatory_protocols = >=TLSv1.2, <=0305
           # Allow only TLS 1.2 and up:
           smtp_tls_mandatory_protocols = >=0x0303

       With Postfix < 3.6 there is no support for a minimum or maximum
       version, and the protocol range is configured via protocol exclusions.
       To require at least TLS 1.0, set "smtp_tls_mandatory_protocols =
       !SSLv2, !SSLv3". Listing the protocols to include, rather than the
       protocols to exclude, is supported, but not recommended.  The exclusion
       syntax more accurately matches the underlying OpenSSL interface.

       When using the exclusion syntax, take care to ensure that the range of
       protocols supported by the Postfix SMTP client is contiguous.  When a
       protocol version is enabled, disabling any higher version implicitly
       disables all versions above that higher version.  Thus, for example:

           smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1.1

       also disables any protocol versions higher than TLSv1.1 leaving only
       "TLSv1" enabled.

       Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.  Disabling this
       protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch
       releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

       While the vast majority of SMTP servers with DANE TLSA records now
       support at least TLS 1.2, a few still only support TLS 1.0.  If you use
       "dane" or "dane-only" it is best not to disable TLSv1, except perhaps
       via the policy table for destinations which you are sure will support
       "TLSv1.2".

       See the documentation of the smtp_tls_policy_maps parameter and
       TLS_README for more information about security levels.

       Example:
       # Preferred syntax with Postfix >= 3.6:
       smtp_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
       # Legacy syntax:
       smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

       This feature is available in Postfix 2.3 and later.

smtp_tls_note_starttls_offer (default: no)
       Log the hostname of a remote SMTP server that offers STARTTLS, when TLS
       is not already enabled for that server.

       The logfile record looks like:

       postfix/smtp[pid]:  Host offered STARTTLS: [name.of.host]

       This feature is available in Postfix 2.2 and later.

smtp_tls_per_site (default: empty)
       Optional lookup tables with the Postfix SMTP client TLS usage policy by
       next-hop destination and by remote SMTP server hostname.  When both
       lookups succeed, the more specific per-site policy (NONE, MUST, etc.)
       overrides the less specific one (MAY), and the more secure per-site
       policy (MUST, etc.) overrides the less secure one (NONE).  With Postfix
       2.3 and later smtp_tls_per_site is strongly discouraged: use
       smtp_tls_policy_maps instead.

       Use of the bare hostname as the per-site table lookup key is
       discouraged. Always use the full destination nexthop (enclosed in []
       with a possible ":port" suffix). A recipient domain or MX-enabled
       transport next-hop with no port suffix may look like a bare hostname,
       but is still a suitable destination.

       Specify a next-hop destination or server hostname on the left-hand
       side; no wildcards are allowed. The next-hop destination is either the
       recipient domain, or the destination specified with a transport(5)
       table, the relayhost parameter, or the relay_transport parameter.  On
       the right hand side specify one of the following keywords:

       NONE   Don't use TLS at all. This overrides a less specific MAY lookup
              result from the alternate host or next-hop lookup key, and
              overrides the global smtp_use_tls, smtp_enforce_tls, and
              smtp_tls_enforce_peername settings.

       MAY    Try to use TLS if the server announces support, otherwise use an
              unencrypted connection. This has less precedence than a more
              specific result (including NONE) from the alternate host or
              next-hop lookup key, and has less precedence than the more
              specific global "smtp_enforce_tls = yes" or
              "smtp_tls_enforce_peername = yes".

       MUST_NOPEERMATCH
              Require TLS encryption, but do not require that the remote SMTP
              server hostname matches the information in the remote SMTP
              server certificate, or that the server certificate was issued by
              a trusted CA. This overrides a less secure NONE or a less
              specific MAY lookup result from the alternate host or next-hop
              lookup key, and overrides the global smtp_use_tls,
              smtp_enforce_tls and smtp_tls_enforce_peername settings.

       MUST   Require TLS encryption, require that the remote SMTP server
              hostname matches the information in the remote SMTP server
              certificate, and require that the remote SMTP server certificate
              was issued by a trusted CA. This overrides a less secure NONE or
              MUST_NOPEERMATCH or a less specific MAY lookup result from the
              alternate host or next-hop lookup key, and overrides the global
              smtp_use_tls, smtp_enforce_tls and smtp_tls_enforce_peername
              settings.

       The above keywords correspond to the "none", "may", "encrypt" and
       "verify" security levels for the new smtp_tls_security_level parameter
       introduced in Postfix 2.3. Starting with Postfix 2.3, and independently
       of how the policy is specified, the smtp_tls_mandatory_ciphers and
       smtp_tls_mandatory_protocols parameters apply when TLS encryption is
       mandatory. Connections for which encryption is optional typically
       enable all "export" grade and better ciphers (see smtp_tls_ciphers and
       smtp_tls_protocols).

       As long as no secure DNS lookup mechanism is available, false hostnames
       in MX or CNAME responses can change the server hostname that Postfix
       uses for TLS policy lookup and server certificate verification. Even
       with a perfect match between the server hostname and the server
       certificate, there is no guarantee that Postfix is connected to the
       right server.  See TLS_README (Closing a DNS loophole with obsolete
       per-site TLS policies) for a possible work-around.

       This feature is available in Postfix 2.2 and later. With Postfix 2.3
       and later use smtp_tls_policy_maps instead.

smtp_tls_policy_maps (default: empty)
       Optional lookup tables with the Postfix SMTP client TLS security policy
       by next-hop destination; when a non-empty value is specified, this
       overrides the obsolete smtp_tls_per_site parameter.  See TLS_README for
       a more detailed discussion of TLS security levels.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       The TLS policy table is indexed by the full next-hop destination, which
       is either the recipient domain, or the verbatim next-hop specified in
       the transport table, $local_transport, $virtual_transport,
       $relay_transport or $default_transport. This includes any enclosing
       square brackets and any non-default destination server port suffix. The
       LMTP socket type prefix (inet: or unix:) is not included in the lookup
       key.

       Only the next-hop domain, or $myhostname with LMTP over UNIX-domain
       sockets, is used as the nexthop name for certificate verification. The
       port and any enclosing square brackets are used in the table lookup
       key, but are not used for server name verification.

       When the lookup key is a domain name without enclosing square brackets
       or any :port suffix (typically the recipient domain), and the full
       domain is not found in the table, just as with the transport(5) table,
       the parent domain starting with a leading "." is matched recursively.
       This allows one to specify a security policy for a recipient domain and
       all its sub-domains.

       The lookup result is a security level, followed by an optional list of
       whitespace and/or comma separated name=value attributes that override
       related main.cf settings. The TLS security levels in order of
       increasing security are:

       none   No TLS. No additional attributes are supported at this level.

       may    Opportunistic TLS. Since sending in the clear is acceptable,
              demanding stronger than default TLS security merely reduces
              interoperability. The optional "ciphers", "exclude", and
              "protocols" attributes (available for opportunistic TLS with
              Postfix >= 2.6) and "connection_reuse" attribute (Postfix >=
              3.4) override the "smtp_tls_ciphers",
              "smtp_tls_exclude_ciphers", "smtp_tls_protocols", and
              "smtp_tls_connection_reuse" configuration parameters. In the
              policy table, multiple ciphers, protocols or excluded ciphers
              must be separated by colons, as attribute values may not contain
              whitespace or commas. When opportunistic TLS handshakes fail,
              Postfix retries the connection with TLS disabled.  This allows
              mail delivery to sites with non-interoperable TLS
              implementations.

       encrypt
              Mandatory TLS encryption. At this level and higher, the optional
              "protocols" attribute overrides the main.cf
              smtp_tls_mandatory_protocols parameter, the optional "ciphers"
              attribute overrides the main.cf smtp_tls_mandatory_ciphers
              parameter, the optional "exclude" attribute (Postfix >= 2.6)
              overrides the main.cf smtp_tls_mandatory_exclude_ciphers
              parameter, and the optional "connection_reuse" attribute
              (Postfix >= 3.4) overrides the main.cf smtp_tls_connection_reuse
              parameter. In the policy table, multiple ciphers, protocols or
              excluded ciphers must be separated by colons, as attribute
              values may not contain whitespace or commas.

       dane   Opportunistic DANE TLS.  The TLS policy for the destination is
              obtained via TLSA records in DNSSEC.  If no TLSA records are
              found, the effective security level used is may.  If TLSA
              records are found, but none are usable, the effective security
              level is encrypt.  When usable TLSA records are obtained for the
              remote SMTP server, the server certificate must match the TLSA
              records.  RFC 7672 (DANE) TLS authentication and DNSSEC support
              is available with Postfix 2.11 and later. The optional
              "connection_reuse" attribute (Postfix >= 3.4) overrides the
              main.cf smtp_tls_connection_reuse parameter.  When the effective
              security level used is may, the optional "ciphers", "exclude",
              and "protocols" attributes (Postfix >= 2.6) override the
              "smtp_tls_ciphers", "smtp_tls_exclude_ciphers", and
              "smtp_tls_protocols" configuration parameters.  When the
              effective security level used is encrypt, the optional
              "ciphers", "exclude", and "protocols" attributes (Postfix >=
              2.6) override the "smtp_tls_mandatory_ciphers",
              "smtp_tls_mandatory_exclude_ciphers", and
              "smtp_tls_mandatory_protocols" configuration parameters.

       dane-only
              Mandatory DANE TLS.  The TLS policy for the destination is
              obtained via TLSA records in DNSSEC.  If no TLSA records are
              found, or none are usable, no connection is made to the server.
              When usable TLSA records are obtained for the remote SMTP
              server, the server certificate must match the TLSA records.  RFC
              7672 (DANE) TLS authentication and DNSSEC support is available
              with Postfix 2.11 and later. The optional "ciphers", "exclude",
              and "protocols" attributes (Postfix >= 2.6) override the
              "smtp_tls_mandatory_ciphers",
              "smtp_tls_mandatory_exclude_ciphers", and
              "smtp_tls_mandatory_protocols" configuration parameters. The
              optional "connection_reuse" attribute (Postfix >= 3.4) overrides
              the main.cf smtp_tls_connection_reuse parameter.

       fingerprint
              Certificate fingerprint verification. Available with Postfix 2.5
              and later. At this security level, there are no trusted
              Certification Authorities. The certificate trust chain,
              expiration date, ... are not checked. Instead, the optional
              "match" attribute, or else the main.cf
              smtp_tls_fingerprint_cert_match parameter, lists the certificate
              fingerprints or the public key fingerprint (Postfix 2.9 and
              later) of the valid server certificate. The digest algorithm
              used to calculate the fingerprint is selected by the
              smtp_tls_fingerprint_digest parameter. Multiple fingerprints can
              be combined with a "|" delimiter in a single match attribute, or
              multiple match attributes can be employed. The ":" character is
              not used as a delimiter as it occurs between each pair of
              fingerprint (hexadecimal) digits. The optional "ciphers",
              "exclude", and "protocols" attributes (Postfix >= 2.6) override
              the "smtp_tls_mandatory_ciphers",
              "smtp_tls_mandatory_exclude_ciphers", and
              "smtp_tls_mandatory_protocols" configuration parameters. The
              optional "connection_reuse" attribute (Postfix >= 3.4) overrides
              the main.cf smtp_tls_connection_reuse parameter.

       verify Mandatory TLS verification.  At this security level, DNS MX
              lookups are trusted to be secure enough, and the name verified
              in the server certificate is usually obtained indirectly via
              unauthenticated DNS MX lookups.  The optional "match" attribute
              overrides the main.cf smtp_tls_verify_cert_match parameter. In
              the policy table, multiple match patterns and strategies must be
              separated by colons.  In practice explicit control over matching
              is more common with the "secure" policy, described below. The
              optional "ciphers", "exclude", and "protocols" attributes
              (Postfix >= 2.6) override the "smtp_tls_mandatory_ciphers",
              "smtp_tls_mandatory_exclude_ciphers", and
              "smtp_tls_mandatory_protocols" configuration parameters. The
              optional "connection_reuse" attribute (Postfix >= 3.4) overrides
              the main.cf smtp_tls_connection_reuse parameter.

       secure Secure-channel TLS. At this security level, DNS MX lookups,
              though potentially used to determine the candidate next-hop
              gateway IP addresses, are not trusted to be secure enough for
              TLS peername verification. Instead, the default name verified in
              the server certificate is obtained directly from the next-hop,
              or is explicitly specified via the optional "match" attribute
              which overrides the main.cf smtp_tls_secure_cert_match
              parameter. In the policy table, multiple match patterns and
              strategies must be separated by colons.  The match attribute is
              most useful when multiple domains are supported by a common
              server: the policy entries for additional domains specify
              matching rules for the primary domain certificate. While
              transport table overrides that route the secondary domains to
              the primary nexthop also allow secure verification, they risk
              delivery to the wrong destination when domains change hands or
              are re-assigned to new gateways. With the "match" attribute
              approach, routing is not perturbed, and mail is deferred if
              verification of a new MX host fails. The optional "ciphers",
              "exclude", and "protocols" attributes (Postfix >= 2.6) override
              the "smtp_tls_mandatory_ciphers",
              "smtp_tls_mandatory_exclude_ciphers", and
              "smtp_tls_mandatory_protocols" configuration parameters. The
              optional "connection_reuse" attribute (Postfix >= 3.4) overrides
              the main.cf smtp_tls_connection_reuse parameter.

       Example:

       /etc/postfix/main.cf:
           smtp_tls_policy_maps = hash:/etc/postfix/tls_policy
           # Postfix 2.5 and later.
           #
           # The default digest is sha256 with Postfix >= 3.6 and
           # compatibility level >= 3.
           #
           smtp_tls_fingerprint_digest = sha256

       /etc/postfix/tls_policy:
           example.edu                 none
           example.mil                 may
           example.gov                 encrypt protocols=TLSv1
           example.com                 verify ciphers=high
           example.net                 secure
           .example.net                secure match=.example.net:example.net
           [mail.example.org]:587      secure match=nexthop
           # Postfix 2.5 and later
           [thumb.example.org]          fingerprint
               match=b6:b4:72:34:e2:59:cd:...:c2:ca:63:0d:4d:cc:2c:7d:84:de:e6:2f
               match=51:e9:af:2e:1e:40:1f:...:64:0a:30:35:2d:09:16:31:5a:eb:82:76

       Note: The "hostname" strategy if listed in a non-default setting of
       smtp_tls_secure_cert_match or in the "match" attribute in the policy
       table can render the "secure" level vulnerable to DNS forgery. Do not
       use the "hostname" strategy for secure-channel configurations in
       environments where DNS security is not assured.

       This feature is available in Postfix 2.3 and later.

smtp_tls_protocols (default: see postconf -d output)
       TLS protocols that the Postfix SMTP client will use with opportunistic
       TLS encryption.  In main.cf the values are separated by whitespace,
       commas or colons. In the policy table "protocols" attribute (see
       smtp_tls_policy_maps) the only valid separator is colon.  An empty
       value means allow all protocols.

       The valid protocol names (see SSL_get_version(3)) are "SSLv2", "SSLv3",
       "TLSv1", "TLSv1.1", "TLSv1.2" and "TLSv1.3".  Starting with Postfix
       3.6, the default value is ">=TLSv1", which sets TLS 1.0 as the lowest
       supported TLS protocol version (see below).  Older releases use the "!"
       exclusion syntax, also described below.

       As of Postfix 3.6, the preferred way to limit the range of acceptable
       protocols is to set the lowest acceptable TLS protocol version and/or
       the highest acceptable TLS protocol version.  To set the lower bound
       include an element of the form: ">=version" where version is either one
       of the TLS protocol names listed above, or a hexadecimal number
       corresponding to the desired TLS protocol version (0301 for TLS 1.0,
       0302 for TLS 1.1, etc.).  For the upper bound, use "<=version".  There
       must be no whitespace between the ">=" or "<=" symbols and the protocol
       name or number.

       Hexadecimal protocol numbers make it possible to specify protocol
       bounds for TLS versions that are known to OpenSSL, but might not be
       known to Postfix.  They cannot be used with the legacy exclusion
       syntax.  Leading "0" or "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent to
       "TLSv1".  Hexadecimal versions unknown to OpenSSL will fail to set the
       upper or lower bound, and a warning will be logged.  Hexadecimal
       versions should only be used when Postfix is linked with some future
       version of OpenSSL that supports TLS 1.4 or later, but Postfix does not
       yet support a symbolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtp_tls_protocols = >=TLSv1, <=0305
           # Allow only TLS 1.0 and up:
           smtp_tls_protocols = >=0x0301

       With Postfix < 3.6 there is no support for a minimum or maximum
       version, and the protocol range is configured via protocol exclusions.
       To require at least TLS 1.0, set "smtp_tls_protocols = !SSLv2, !SSLv3".
       Listing the protocols to include, rather than protocols to exclude, is
       supported, but not recommended.  The exclusion form more accurately
       matches the underlying OpenSSL interface.

       When using the exclusion syntax, take care to ensure that the range of
       protocols advertised by an SSL/TLS client is contiguous.  When a
       protocol version is enabled, disabling any higher version implicitly
       disables all versions above that higher version.  Thus, for example:

           smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1.1
       also disables any protocols version higher than TLSv1.1 leaving only
       "TLSv1" enabled.

       Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.  Disabling this
       protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch
       releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

       Example:
       # Preferred syntax with Postfix >= 3.6:
       smtp_tls_protocols = >=TLSv1, <=TLSv1.3
       # Legacy syntax:
       smtp_tls_protocols = !SSLv2, !SSLv3

       This feature is available in Postfix 2.6 and later.

smtp_tls_scert_verifydepth (default: 9)
       The verification depth for remote SMTP server certificates. A depth of
       1 is sufficient if the issuing CA is listed in a local CA file.

       The default verification depth is 9 (the OpenSSL default) for
       compatibility with earlier Postfix behavior. Prior to Postfix 2.5, the
       default value was 5, but the limit was not actually enforced. If you
       have set this to a lower non-default value, certificates with longer
       trust chains may now fail to verify. Certificate chains with 1 or 2 CAs
       are common, deeper chains are more rare and any number between 5 and 9
       should suffice in practice. You can choose a lower number if, for
       example, you trust certificates directly signed by an issuing CA but
       not any CAs it delegates to.

       This feature is available in Postfix 2.2 and later.

smtp_tls_secure_cert_match (default: nexthop, dot-nexthop)
       How the Postfix SMTP client verifies the server certificate peername
       for the "secure" TLS security level. In a "secure" TLS policy table
       ($smtp_tls_policy_maps) entry the optional "match" attribute overrides
       this main.cf setting.

       This parameter specifies one or more patterns or strategies separated
       by commas, whitespace or colons.  In the policy table the only valid
       separator is the colon character.

       For a description of the pattern and strategy syntax see the
       smtp_tls_verify_cert_match parameter. The "hostname" strategy should be
       avoided in this context, as in the absence of a secure global DNS,
       using the results of MX lookups in certificate verification is not
       immune to active (man-in-the-middle) attacks on DNS.

       Sample main.cf setting:

           smtp_tls_secure_cert_match = nexthop

       Sample policy table override:

           example.net     secure match=example.com:.example.com
           .example.net    secure match=example.com:.example.com

       This feature is available in Postfix 2.3 and later.

smtp_tls_security_level (default: empty)
       The default SMTP TLS security level for the Postfix SMTP client.  When
       a non-empty value is specified, this overrides the obsolete parameters
       smtp_use_tls, smtp_enforce_tls, and smtp_tls_enforce_peername; when no
       value is specified for smtp_tls_enforce_peername or the obsolete
       parameters, the default SMTP TLS security level is none.

       Specify one of the following security levels:

       none   No TLS. TLS will not be used unless enabled for specific
              destinations via smtp_tls_policy_maps.

       may    Opportunistic TLS. Use TLS if this is supported by the remote
              SMTP server, otherwise use plaintext. Since sending in the clear
              is acceptable, demanding stronger than default TLS security
              merely reduces interoperability.  The "smtp_tls_ciphers" and
              "smtp_tls_protocols" (Postfix >= 2.6) configuration parameters
              provide control over the protocols and cipher grade used with
              opportunistic TLS.  With earlier releases the opportunistic TLS
              cipher grade is always "export" and no protocols are disabled.
              When TLS handshakes fail, the connection is retried with TLS
              disabled.  This allows mail delivery to sites with
              non-interoperable TLS implementations.

       encrypt
              Mandatory TLS encryption. Since a minimum level of security is
              intended, it is reasonable to be specific about sufficiently
              secure protocol versions and ciphers. At this security level and
              higher, the main.cf parameters smtp_tls_mandatory_protocols and
              smtp_tls_mandatory_ciphers specify the TLS protocols and minimum
              cipher grade which the administrator considers secure enough for
              mandatory encrypted sessions. This security level is not an
              appropriate default for systems delivering mail to the Internet.

       dane   Opportunistic DANE TLS.  At this security level, the TLS policy
              for the destination is obtained via DNSSEC.  For TLSA policy to
              be in effect, the destination domain's containing DNS zone must
              be signed and the Postfix SMTP client's operating system must be
              configured to send its DNS queries to a recursive DNS nameserver
              that is able to validate the signed records.  Each MX host's DNS
              zone should also be signed, and should publish DANE TLSA (RFC
              7672) records that specify how that MX host's TLS certificate is
              to be verified.  TLSA records do not preempt the normal SMTP MX
              host selection algorithm, if some MX hosts support TLSA and
              others do not, TLS security will vary from delivery to delivery.
              It is up to the domain owner to configure their MX hosts and
              their DNS sensibly.  To configure the Postfix SMTP client for
              DNSSEC lookups see the documentation for the
              smtp_dns_support_level main.cf parameter.  When DNSSEC-validated
              TLSA records are not found the effective tls security level is
              "may".  When TLSA records are found, but are all unusable the
              effective security level is "encrypt".  For purposes of protocol
              and cipher selection, the "dane" security level is treated like
              a "mandatory" TLS security level, and weak ciphers and protocols
              are disabled.  Since DANE authenticates server certificates the
              "aNULL" cipher-suites are transparently excluded at this level,
              no need to configure this manually.  RFC 7672 (DANE) TLS
              authentication is available with Postfix 2.11 and later.

       dane-only
              Mandatory DANE TLS.  This is just like "dane" above, but DANE
              TLSA authentication is required.  There is no fallback to "may"
              or "encrypt" when TLSA records are missing or unusable.  RFC
              7672 (DANE) TLS authentication is available with Postfix 2.11
              and later.

       fingerprint
              Certificate fingerprint verification.  At this security level,
              there are no trusted Certification Authorities.  The certificate
              trust chain, expiration date, etc., are not checked. Instead,
              the smtp_tls_fingerprint_cert_match parameter lists the
              certificate fingerprint or public key fingerprint (Postfix 2.9
              and later) of the valid server certificate. The digest algorithm
              used to calculate the fingerprint is selected by the
              smtp_tls_fingerprint_digest parameter. Available with Postfix
              2.5 and later.

       verify Mandatory TLS verification. At this security level, DNS MX
              lookups are trusted to be secure enough, and the name verified
              in the server certificate is usually obtained indirectly via
              unauthenticated DNS MX lookups. The smtp_tls_verify_cert_match
              parameter controls how the server name is verified. In practice
              explicit control over matching is more common at the "secure"
              level, described below. This security level is not an
              appropriate default for systems delivering mail to the Internet.

       secure Secure-channel TLS.  At this security level, DNS MX lookups,
              though potentially used to determine the candidate next-hop
              gateway IP addresses, are not trusted to be secure enough for
              TLS peername verification. Instead, the default name verified in
              the server certificate is obtained from the next-hop domain as
              specified in the smtp_tls_secure_cert_match configuration
              parameter. The default matching rule is that a server
              certificate matches when its name is equal to or is a sub-domain
              of the nexthop domain. This security level is not an appropriate
              default for systems delivering mail to the Internet.

       Examples:

       # No TLS. Formerly: smtp_use_tls=no and smtp_enforce_tls=no.
       smtp_tls_security_level = none

       # Opportunistic TLS.
       smtp_tls_security_level = may
       # Do not tweak opportunistic ciphers or protocols unless it is essential
       # to do so (if a security vulnerability is found in the SSL library that
       # can be mitigated by disabling a particular protocol or raising the
       # cipher grade).
       smtp_tls_ciphers = medium
       smtp_tls_protocols = >=TLSv1
       # Legacy (Postfix < 3.6) syntax:
       smtp_tls_protocols = !SSLv2, !SSLv3

       # Mandatory (high-grade) TLS encryption.
       smtp_tls_security_level = encrypt
       smtp_tls_mandatory_ciphers = high

       # Authenticated TLS 1.2 or better matching the nexthop domain or a
       # subdomain.
       smtp_tls_security_level = secure
       smtp_tls_mandatory_ciphers = high
       smtp_tls_mandatory_protocols = >=TLSv1.2
       smtp_tls_secure_cert_match = nexthop, dot-nexthop

       # Certificate fingerprint verification (Postfix >= 2.5).
       # The CA-less "fingerprint" security level only scales to a limited
       # number of destinations. As a global default rather than a per-site
       # setting, this is practical only when mail for all recipients is sent
       # to a central mail hub.
       relayhost = [mailhub.example.com]
       smtp_tls_security_level = fingerprint
       smtp_tls_mandatory_protocols = >=TLSv1.2
       smtp_tls_mandatory_ciphers = high
       smtp_tls_fingerprint_cert_match =
           3D:95:34:51:...:40:99:C0:C1
           EC:3B:2D:B0:...:A3:9D:72:F6

       This feature is available in Postfix 2.3 and later.

smtp_tls_servername (default: empty)
       Optional name to send to the remote SMTP server in the TLS Server Name
       Indication (SNI) extension.  The SNI extension is always on when DANE
       is used to authenticate the server, and in that case the SNI name sent
       is the one required by RFC7672 and this parameter is ignored.

       Some SMTP servers use the received SNI name to select an appropriate
       certificate chain to present to the client.  While this may improve
       interoperability with such servers, it may reduce interoperability with
       other servers that choose to abort the connection when they don't have
       a certificate chain configured for the requested name.  Such servers
       should select a default certificate chain and continue the handshake,
       but some may not.  Therefore, absent DANE, no SNI name is sent by
       default.

       The SNI name must be either a valid DNS hostname, or else one of the
       special values hostname or nexthop, which select either the remote
       hostname or the nexthop domain respectively.  DNS names for SNI must be
       in A-label (punycode) form.  Invalid DNS names log a configuration
       error warning and mail delivery is deferred.

       Except when using a relayhost to forward all email, the only sensible
       non-empty main.cf setting for this parameter is hostname.  Other
       non-empty values are only practical on a per-destination basis via the
       servername attribute of the Postfix TLS policy table.  When in doubt,
       leave this parameter empty, and configure per-destination SNI as
       needed.

       This feature is available in Postfix 3.4 and later.

smtp_tls_session_cache_database (default: empty)
       Name of the file containing the optional Postfix SMTP client TLS
       session cache. Specify a database type that supports enumeration, such
       as btree or sdbm; there is no need to support concurrent access.  The
       file is created if it does not exist. The smtp(8) daemon does not use
       this parameter directly, rather the cache is implemented indirectly in
       the tlsmgr(8) daemon. This means that per-smtp-instance master.cf
       overrides of this parameter are not effective.  Note that each of the
       cache databases supported by tlsmgr(8) daemon:
       $smtpd_tls_session_cache_database, $smtp_tls_session_cache_database
       (and with Postfix 2.3 and later $lmtp_tls_session_cache_database),
       needs to be stored separately. It is not at this time possible to store
       multiple caches in a single database.

       Note: dbm databases are not suitable. TLS session objects are too
       large.

       As of version 2.5, Postfix no longer uses root privileges when opening
       this file. The file should now be stored under the Postfix-owned
       data_directory. As a migration aid, an attempt to open the file under a
       non-Postfix directory is redirected to the Postfix-owned
       data_directory, and a warning is logged.

       Example:

       smtp_tls_session_cache_database = btree:/var/db/postfix/smtp_scache

       This feature is available in Postfix 2.2 and later.

smtp_tls_session_cache_timeout (default: 3600s)
       The expiration time of Postfix SMTP client TLS session cache
       information.  A cache cleanup is performed periodically every
       $smtp_tls_session_cache_timeout seconds. As with
       $smtp_tls_session_cache_database, this parameter is implemented in the
       tlsmgr(8) daemon and therefore per-smtp-instance master.cf overrides
       are not possible.

       As of Postfix 2.11 this setting cannot exceed 100 days.  If set <= 0,
       session caching is disabled.  If set to a positive value less than 2
       minutes, the minimum value of 2 minutes is used instead.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.2 and later.

smtp_tls_trust_anchor_file (default: empty)
       Zero or more PEM-format files with trust-anchor certificates and/or
       public keys.  If the parameter is not empty the root CAs in CAfile and
       CApath are no longer trusted.  Rather, the Postfix SMTP client will
       only trust certificate-chains signed by one of the trust-anchors
       contained in the chosen files.  The specified trust-anchor certificates
       and public keys are not subject to expiration, and need not be
       (self-signed) root CAs.  They may, if desired, be intermediate
       certificates. Therefore, these certificates also may be found "in the
       middle" of the trust chain presented by the remote SMTP server, and any
       untrusted issuing parent certificates will be ignored.  Specify a list
       of pathnames separated by comma or whitespace.

       Whether specified in main.cf, or on a per-destination basis, the
       trust-anchor PEM file must be accessible to the Postfix SMTP client in
       the chroot jail if applicable.  The trust-anchor file should contain
       only certificates and public keys, no private key material, and must be
       readable by the non-privileged $mail_owner user.  This allows
       destinations to be bound to a set of specific CAs or public keys
       without trusting the same CAs for all destinations.

       The main.cf parameter supports single-purpose Postfix installations
       that send mail to a fixed set of SMTP peers.  At most sites, if
       trust-anchor files are used at all, they will be specified on a
       per-destination basis via the "tafile" attribute of the "verify" and
       "secure" levels in smtp_tls_policy_maps.

       The underlying mechanism is in support of RFC 7672 (DANE TLSA), which
       defines mechanisms for an SMTP client MTA to securely determine server
       TLS certificates via DNS.

       If you want your trust anchors to be public keys, with OpenSSL you can
       extract a single PEM public key from a PEM X.509 file containing a
       single certificate, as follows:

           $ openssl x509 -in cert.pem -out ta-key.pem -noout -pubkey

       This feature is available in Postfix 2.11 and later.

smtp_tls_verify_cert_match (default: hostname)
       How the Postfix SMTP client verifies the server certificate peername
       for the "verify" TLS security level. In a "verify" TLS policy table
       ($smtp_tls_policy_maps) entry the optional "match" attribute overrides
       this main.cf setting.

       This parameter specifies one or more patterns or strategies separated
       by commas, whitespace or colons.  In the policy table the only valid
       separator is the colon character.

       Patterns specify domain names, or domain name suffixes:

       example.com
              Match the example.com domain, i.e. one of the names in the
              server certificate must be example.com.  Upper and lower case
              distinctions are ignored.

       .example.com
              Match subdomains of the example.com domain, i.e. match a name in
              the server certificate that consists of a non-zero number of
              labels followed by a .example.com suffix. Case distinctions are
              ignored.

       Strategies specify a transformation from the next-hop domain to the
       expected name in the server certificate:

       nexthop
              Match against the next-hop domain, which is either the recipient
              domain, or the transport next-hop configured for the domain
              stripped of any optional socket type prefix, enclosing square
              brackets and trailing port. When MX lookups are not suppressed,
              this is the original nexthop domain prior to the MX lookup, not
              the result of the MX lookup. For LMTP delivery via UNIX-domain
              sockets, the verified next-hop name is $myhostname.  This
              strategy is suitable for use with the "secure" policy. Case is
              ignored.

       dot-nexthop
              As above, but match server certificate names that are subdomains
              of the next-hop domain. Case is ignored.

       hostname
              Match against the hostname of the server, often obtained via an
              unauthenticated DNS MX lookup. For LMTP delivery via UNIX-domain
              sockets, the verified name is $myhostname. This matches the
              verification strategy of the "MUST" keyword in the obsolete
              smtp_tls_per_site table, and is suitable for use with the
              "verify" security level. When the next-hop name is enclosed in
              square brackets to suppress MX lookups, the "hostname" strategy
              is the same as the "nexthop" strategy. Case is ignored.

       Sample main.cf setting:

       smtp_tls_verify_cert_match = hostname, nexthop, dot-nexthop

       Sample policy table override:

       example.com     verify  match=hostname:nexthop
       .example.com    verify  match=example.com:.example.com:hostname

       This feature is available in Postfix 2.3 and later.

smtp_tls_wrappermode (default: no)
       Request that the Postfix SMTP client connects using the
       SUBMISSIONS/SMTPS protocol instead of using the STARTTLS command.

       This mode requires "smtp_tls_security_level = encrypt" or stronger.

       Example: deliver all remote mail via a provider's server
       "mail.example.com".

       /etc/postfix/main.cf:
           # Client-side SMTPS requires "encrypt" or stronger.
           smtp_tls_security_level = encrypt
           smtp_tls_wrappermode = yes
           # The [] suppress MX lookups.
           relayhost = [mail.example.com]:465

       More examples are in TLS_README, including examples for older Postfix
       versions.

       This feature is available in Postfix 3.0 and later.

smtp_use_tls (default: no)
       Opportunistic mode: use TLS when a remote SMTP server announces
       STARTTLS support, otherwise send the mail in the clear. Beware: some
       SMTP servers offer STARTTLS even if it is not configured.  With Postfix
       < 2.3, if the TLS handshake fails, and no other server is available,
       delivery is deferred and mail stays in the queue. If this is a concern
       for you, use the smtp_tls_per_site feature instead.

       This feature is available in Postfix 2.2 and later. With Postfix 2.3
       and later use smtp_tls_security_level instead.

smtp_xforward_timeout (default: 300s)
       The Postfix SMTP client time limit for sending the XFORWARD command,
       and for receiving the remote SMTP server response.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

smtpd_authorized_verp_clients (default: $authorized_verp_clients)
       What remote SMTP clients are allowed to specify the XVERP command.
       This command requests that mail be delivered one recipient at a time
       with a per recipient return address.

       By default, no clients are allowed to specify XVERP.

       This parameter was renamed with Postfix version 2.1. The default value
       is backwards compatible with Postfix version 2.0.

       Specify a list of network/netmask patterns, separated by commas and/or
       whitespace. The mask specifies the number of bits in the network part
       of a host address. You can also specify hostnames or .domain names (the
       initial dot causes the domain to match any name below it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is
       replaced by its contents; a "type:table" lookup table is matched when a
       table entry matches a lookup string (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude an address or network block from the list. The
       form "!/file/name" is supported only in Postfix version 2.4 and later.

       Note: IP version 6 address information must be specified inside [] in
       the smtpd_authorized_verp_clients value, and in files specified with
       "/file/name".  IP version 6 addresses contain the ":" character, and
       would otherwise be confused with a "type:table" pattern.

smtpd_authorized_xclient_hosts (default: empty)
       What remote SMTP clients are allowed to use the XCLIENT feature.  This
       command overrides remote SMTP client information that is used for
       access control. Typical use is for SMTP-based content filters,
       fetchmail-like programs, or SMTP server access rule testing. See the
       XCLIENT_README document for details.

       This feature is available in Postfix 2.1 and later.

       By default, no clients are allowed to specify XCLIENT.

       Specify a list of network/netmask patterns, separated by commas and/or
       whitespace. The mask specifies the number of bits in the network part
       of a host address. You can also specify hostnames or .domain names (the
       initial dot causes the domain to match any name below it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is
       replaced by its contents; a "type:table" lookup table is matched when a
       table entry matches a lookup string (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude an address or network block from the list. The
       form "!/file/name" is supported only in Postfix version 2.4 and later.

       Note: IP version 6 address information must be specified inside [] in
       the smtpd_authorized_xclient_hosts value, and in files specified with
       "/file/name".  IP version 6 addresses contain the ":" character, and
       would otherwise be confused with a "type:table" pattern.

smtpd_authorized_xforward_hosts (default: empty)
       What remote SMTP clients are allowed to use the XFORWARD feature.  This
       command forwards information that is used to improve logging after
       SMTP-based content filters. See the XFORWARD_README document for
       details.

       This feature is available in Postfix 2.1 and later.

       By default, no clients are allowed to specify XFORWARD.

       Specify a list of network/netmask patterns, separated by commas and/or
       whitespace. The mask specifies the number of bits in the network part
       of a host address. You can also specify hostnames or .domain names (the
       initial dot causes the domain to match any name below it),
       "/file/name" or "type:table" patterns.  A "/file/name" pattern is
       replaced by its contents; a "type:table" lookup table is matched when a
       table entry matches a lookup string (the lookup result is ignored).
       Continue long lines by starting the next line with whitespace. Specify
       "!pattern" to exclude an address or network block from the list. The
       form "!/file/name" is supported only in Postfix version 2.4 and later.

       Note: IP version 6 address information must be specified inside [] in
       the smtpd_authorized_xforward_hosts value, and in files specified with
       "/file/name".  IP version 6 addresses contain the ":" character, and
       would otherwise be confused with a "type:table" pattern.

smtpd_banner (default: $myhostname ESMTP $mail_name)
       The text that follows the 220 status code in the SMTP greeting banner.
       Some people like to see the mail version advertised. By default,
       Postfix shows no version.

       You MUST specify $myhostname at the start of the text. This is required
       by the SMTP protocol.

       Example:

       smtpd_banner = $myhostname ESMTP $mail_name ($mail_version)

smtpd_client_auth_rate_limit (default: 0)
       The maximal number of AUTH commands that any client is allowed to send
       to this service per time unit, regardless of whether or not Postfix
       actually accepts those commands.  The time unit is specified with the
       anvil_rate_time_unit configuration parameter.

       By default, there is no limit on the number of AUTH commands that a
       client may send.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be
       used to regulate legitimate mail traffic.

       This feature is available in Postfix 3.1 and later.

smtpd_client_connection_count_limit (default: 50)
       How many simultaneous connections any client is allowed to make to this
       service.  By default, the limit is set to half the default process
       limit value.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be
       used to regulate legitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

smtpd_client_connection_rate_limit (default: 0)
       The maximal number of connection attempts any client is allowed to make
       to this service per time unit.  The time unit is specified with the
       anvil_rate_time_unit configuration parameter.

       By default, a client can make as many connections per time unit as
       Postfix can accept.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be
       used to regulate legitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

       Example:

       smtpd_client_connection_rate_limit = 1000

smtpd_client_event_limit_exceptions (default: $mynetworks)
       Clients that are excluded from smtpd_client_*_count/rate_limit
       restrictions. See the mynetworks parameter description for the
       parameter value syntax.

       By default, clients in trusted networks are excluded. Specify a list of
       network blocks, hostnames or .domain names (the initial dot causes the
       domain to match any name below it).

       Note: IP version 6 address information must be specified inside [] in
       the smtpd_client_event_limit_exceptions value, and in files specified
       with "/file/name".  IP version 6 addresses contain the ":" character,
       and would otherwise be confused with a "type:table" pattern.

       Pattern matching of domain names is controlled by the presence or
       absence of "smtpd_client_event_limit_exceptions" in the
       parent_domain_matches_subdomains parameter value (Postfix 3.0 and
       later).

       This feature is available in Postfix 2.2 and later.

smtpd_client_message_rate_limit (default: 0)
       The maximal number of message delivery requests that any client is
       allowed to make to this service per time unit, regardless of whether or
       not Postfix actually accepts those messages.  The time unit is
       specified with the anvil_rate_time_unit configuration parameter.

       By default, a client can send as many message delivery requests per
       time unit as Postfix can accept.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be
       used to regulate legitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

       Example:

       smtpd_client_message_rate_limit = 1000

smtpd_client_new_tls_session_rate_limit (default: 0)
       The maximal number of new (i.e., uncached) TLS sessions that a remote
       SMTP client is allowed to negotiate with this service per time unit.
       The time unit is specified with the anvil_rate_time_unit configuration
       parameter.

       By default, a remote SMTP client can negotiate as many new TLS sessions
       per time unit as Postfix can accept.

       To disable this feature, specify a limit of 0. Otherwise, specify a
       limit that is at least the per-client concurrent session limit, or else
       legitimate client sessions may be rejected.

       WARNING: The purpose of this feature is to limit abuse. It must not be
       used to regulate legitimate mail traffic.

       This feature is available in Postfix 2.3 and later.

       Example:

       smtpd_client_new_tls_session_rate_limit = 100

smtpd_client_port_logging (default: no)
       Enable logging of the remote SMTP client port in addition to the
       hostname and IP address. The logging format is "host[address]:port".

       This feature is available in Postfix 2.5 and later.

smtpd_client_recipient_rate_limit (default: 0)
       The maximal number of recipient addresses that any client is allowed to
       send to this service per time unit, regardless of whether or not
       Postfix actually accepts those recipients.  The time unit is specified
       with the anvil_rate_time_unit configuration parameter.

       By default, a client can send as many recipient addresses per time unit
       as Postfix can accept.

       To disable this feature, specify a limit of 0.

       WARNING: The purpose of this feature is to limit abuse. It must not be
       used to regulate legitimate mail traffic.

       This feature is available in Postfix 2.2 and later.

       Example:

       smtpd_client_recipient_rate_limit = 1000

smtpd_client_restrictions (default: empty)
       Optional restrictions that the Postfix SMTP server applies in the
       context of a client connection request.  See SMTPD_ACCESS_README,
       section "Delayed evaluation of SMTP access restriction lists" for a
       discussion of evaluation context and time.

       The default is to allow all connection requests.

       Specify a list of restrictions, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace.
       Restrictions are applied in the order as specified; the first
       restriction that matches wins.

       The following restrictions are specific to client hostname or client
       network address information.

       check_ccert_access type:table
              By default use the remote SMTP client certificate fingerprint or
              the public key fingerprint (Postfix 2.9 and later) as the lookup
              key for the specified access(5) database; with Postfix version
              2.2, also require that the remote SMTP client certificate is
              verified successfully.  The fingerprint digest algorithm is
              configurable via the smtpd_tls_fingerprint_digest parameter
              (hard-coded as md5 prior to Postfix version 2.5).  This feature
              requires "smtpd_tls_ask_ccert = yes" and is available with
              Postfix version 2.2 and later.
              The default algorithm is sha256 with Postfix >= 3.6 and the
              compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
              the default algorithm is md5.  The best-practice algorithm is
              now sha256. Recent advances in hash function cryptanalysis have
              led to md5 and sha1 being deprecated in favor of sha256.
              However, as long as there are no known "second pre-image"
              attacks against the older algorithms, their use in this context,
              though not recommended, is still likely safe.
              Alternatively, check_ccert_access accepts an explicit search
              order (Postfix 3.5 and later). The default search order as
              described above corresponds with:
              check_ccert_access { type:table, { search_order =
              cert_fingerprint, pubkey_fingerprint } }
              The commas are optional.

       check_client_access type:table
              Search the specified access database for the client hostname,
              parent domains, client IP address, or networks obtained by
              stripping least significant octets. See the access(5) manual
              page for details.

       check_client_a_access type:table
              Search the specified access(5) database for the IP addresses for
              the client hostname, and execute the corresponding action.
              Note: a result of "OK" is not allowed for safety reasons.
              Instead, use DUNNO in order to exclude specific hosts from
              denylists.  This feature is available in Postfix 3.0 and later.

       check_client_mx_access type:table
              Search the specified access(5) database for the MX hosts for the
              client hostname, and execute the corresponding action.  If no MX
              record is found, look up A or AAAA records, just like the
              Postfix SMTP client would. Note: a result of "OK" is not allowed
              for safety reasons. Instead, use DUNNO in order to exclude
              specific hosts from denylists.  This feature is available in
              Postfix 2.7 and later.

       check_client_ns_access type:table
              Search the specified access(5) database for the DNS servers for
              the client hostname, and execute the corresponding action.
              Note: a result of "OK" is not allowed for safety reasons.
              Instead, use DUNNO in order to exclude specific hosts from
              denylists.  This feature is available in Postfix 2.7 and later.

       check_reverse_client_hostname_access type:table
              Search the specified access database for the unverified reverse
              client hostname, parent domains, client IP address, or networks
              obtained by stripping least significant octets. See the
              access(5) manual page for details.  Note: a result of "OK" is
              not allowed for safety reasons.  Instead, use DUNNO in order to
              exclude specific hosts from denylists.  This feature is
              available in Postfix 2.6 and later.

       check_reverse_client_hostname_a_access type:table
              Search the specified access(5) database for the IP addresses for
              the unverified reverse client hostname, and execute the
              corresponding action.  Note: a result of "OK" is not allowed for
              safety reasons.  Instead, use DUNNO in order to exclude specific
              hosts from denylists.  This feature is available in Postfix 3.0
              and later.

       check_reverse_client_hostname_mx_access type:table
              Search the specified access(5) database for the MX hosts for the
              unverified reverse client hostname, and execute the
              corresponding action.  If no MX record is found, look up A or
              AAAA records, just like the Postfix SMTP client would.  Note: a
              result of "OK" is not allowed for safety reasons.  Instead, use
              DUNNO in order to exclude specific hosts from denylists.  This
              feature is available in Postfix 2.7 and later.

       check_reverse_client_hostname_ns_access type:table
              Search the specified access(5) database for the DNS servers for
              the unverified reverse client hostname, and execute the
              corresponding action.  Note: a result of "OK" is not allowed for
              safety reasons.  Instead, use DUNNO in order to exclude specific
              hosts from denylists.  This feature is available in Postfix 2.7
              and later.

       check_sasl_access type:table
              Use the remote SMTP client SASL user name as the lookup key for
              the specified access(5) database. The lookup key has the form
              "username@domainname" when the smtpd_sasl_local_domain parameter
              value is non-empty.  Unlike the check_client_access feature,
              check_sasl_access does not perform matches of parent domains or
              IP subnet ranges.  This feature is available with Postfix
              version 2.11 and later.

       permit_inet_interfaces
              Permit the request when the client IP address matches
              $inet_interfaces.

       permit_mynetworks
              Permit the request when the client IP address matches any
              network or network address listed in  $mynetworks.

       permit_sasl_authenticated
              Permit the request when the client is successfully authenticated
              via the RFC 4954 (AUTH) protocol.

       permit_tls_all_clientcerts
              Permit the request when the remote SMTP client certificate is
              verified successfully.  This option must be used only if a
              special CA issues the certificates and only this CA is listed as
              a trusted CA. Otherwise, clients with a third-party certificate
              would also be allowed to relay.  Specify "tls_append_default_CA
              = no" when the trusted CA is specified with smtpd_tls_CAfile or
              smtpd_tls_CApath, to prevent Postfix from appending the
              system-supplied default CAs.  This feature requires
              "smtpd_tls_ask_ccert = yes" and is available with Postfix
              version 2.2 and later.

       permit_tls_clientcerts
              Permit the request when the remote SMTP client certificate
              fingerprint or public key fingerprint (Postfix 2.9 and later) is
              listed in $relay_clientcerts.  The fingerprint digest algorithm
              is configurable via the smtpd_tls_fingerprint_digest parameter
              (hard-coded as md5 prior to Postfix version 2.5).  This feature
              requires "smtpd_tls_ask_ccert = yes" and is available with
              Postfix version 2.2 and later.
              The default algorithm is sha256 with Postfix >= 3.6 and the
              compatibility_level set to 3.6 or higher. With Postfix <= 3.5,
              the default algorithm is md5.  The best-practice algorithm is
              now sha256. Recent advances in hash function cryptanalysis have
              led to md5 and sha1 being deprecated in favor of sha256.
              However, as long as there are no known "second pre-image"
              attacks against the older algorithms, their use in this context,
              though not recommended, is still likely safe.

       reject_rbl_client rbl_domain=d.d.d.d
              Reject the request when the reversed client network address is
              listed with the A record "d.d.d.d" under rbl_domain (Postfix
              version 2.1 and later only).  Each "d" is a number, or a pattern
              inside "[]" that contains one or more ";"-separated numbers or
              number..number ranges (Postfix version 2.8 and later).  If no
              "=d.d.d.d" is specified, reject the request when the reversed
              client network address is listed with any A record under
              rbl_domain.
              The maps_rbl_reject_code parameter specifies the response code
              for rejected requests (default:  554), the default_rbl_reply
              parameter specifies the default server reply, and the
              rbl_reply_maps  parameter specifies tables with server replies
              indexed by rbl_domain.  This feature is available in Postfix 2.0
              and later.

       permit_dnswl_client dnswl_domain=d.d.d.d
              Accept the request when the reversed client network address is
              listed with the A record "d.d.d.d" under dnswl_domain.  Each "d"
              is a number, or a pattern inside "[]" that contains one or more
              ";"-separated numbers or number..number ranges.  If no
              "=d.d.d.d" is specified, accept the request when the reversed
              client network address is listed with any A record under
              dnswl_domain.
              For safety, permit_dnswl_client is silently ignored when it
              would override reject_unauth_destination.  The result is
              DEFER_IF_REJECT when allowlist lookup fails.  This feature is
              available in Postfix 2.8 and later.

       reject_rhsbl_client rbl_domain=d.d.d.d
              Reject the request when the client hostname is listed with the A
              record "d.d.d.d" under rbl_domain (Postfix version 2.1 and later
              only).  Each "d" is a number, or a pattern inside "[]" that
              contains one or more ";"-separated numbers or number..number
              ranges (Postfix version 2.8 and later).  If no "=d.d.d.d" is
              specified, reject the request when the client hostname is listed
              with any A record under rbl_domain. See the reject_rbl_client
              description above for additional RBL related configuration
              parameters.  This feature is available in Postfix 2.0 and later;
              with Postfix version 2.8 and later, reject_rhsbl_reverse_client
              will usually produce better results.

       permit_rhswl_client rhswl_domain=d.d.d.d
              Accept the request when the client hostname is listed with the A
              record "d.d.d.d" under rhswl_domain.  Each "d" is a number, or a
              pattern inside "[]" that contains one or more ";"-separated
              numbers or number..number ranges. If no "=d.d.d.d" is specified,
              accept the request when the client hostname is listed with any A
              record under rhswl_domain.
              Caution: client name allowlisting is fragile, since the client
              name lookup can fail due to temporary outages.  Client name
              allowlisting should be used only to reduce false positives in
              e.g.  DNS-based blocklists, and not for making access rule
              exceptions.
              For safety, permit_rhswl_client is silently ignored when it
              would override reject_unauth_destination.  The result is
              DEFER_IF_REJECT when allowlist lookup fails.  This feature is
              available in Postfix 2.8 and later.

       reject_rhsbl_reverse_client rbl_domain=d.d.d.d
              Reject the request when the unverified reverse client hostname
              is listed with the A record "d.d.d.d" under rbl_domain.  Each
              "d" is a number, or a pattern inside "[]" that contains one or
              more ";"-separated numbers or number..number ranges.  If no
              "=d.d.d.d" is specified, reject the request when the unverified
              reverse client hostname is listed with any A record under
              rbl_domain. See the reject_rbl_client description above for
              additional RBL related configuration parameters.  This feature
              is available in Postfix 2.8 and later.

       reject_unknown_client_hostname (with Postfix < 2.3:
       reject_unknown_client)
              Reject the request when 1) the client IP address->name mapping
              fails, or 2) the name->address mapping fails, or 3) the
              name->address mapping does not match the client IP address.
              This is a stronger restriction than the
              reject_unknown_reverse_client_hostname feature, which triggers
              only under condition 1) above.
              The unknown_client_reject_code parameter specifies the response
              code for rejected requests (default: 450). The reply is always
              450 in case the address->name or name->address lookup failed due
              to a temporary problem.

       reject_unknown_reverse_client_hostname
              Reject the request when the client IP address has no
              address->name mapping.
              This is a weaker restriction than the
              reject_unknown_client_hostname feature, which requires not only
              that the address->name and name->address mappings exist, but
              also that the two mappings reproduce the client IP address.
              The unknown_client_reject_code parameter specifies the response
              code for rejected requests (default: 450).  The reply is always
              450 in case the address->name lookup failed due to a temporary
              problem.
              This feature is available in Postfix 2.3 and later.

       In addition, you can use any of the following generic restrictions.
       These restrictions are applicable in any SMTP command context.

       check_policy_service servername
              Query the specified policy server. See the SMTPD_POLICY_README
              document for details. This feature is available in Postfix 2.1
              and later.

       defer  Defer the request. The client is told to try again later. This
              restriction is useful at the end of a restriction list, to make
              the default policy explicit.
              The defer_code parameter specifies the SMTP server reply code
              (default: 450).

       defer_if_permit
              Defer the request if some later restriction would result in an
              explicit or implicit PERMIT action.  This is useful when a
              denylisting feature fails due to a temporary problem.  This
              feature is available in Postfix version 2.1 and later.

       defer_if_reject
              Defer the request if some later restriction would result in a
              REJECT action.  This is useful when an allowlisting feature
              fails due to a temporary problem.  This feature is available in
              Postfix version 2.1 and later.

       permit Permit the request. This restriction is useful at the end of a
              restriction list, to make the default policy explicit.

       reject_multi_recipient_bounce
              Reject the request when the envelope sender is the null address,
              and the message has multiple envelope recipients. This usage has
              rare but legitimate applications: under certain conditions,
              multi-recipient mail that was posted with the DSN option
              NOTIFY=NEVER may be forwarded with the null sender address.
              Note: this restriction can only work reliably when used in
              smtpd_data_restrictions or smtpd_end_of_data_restrictions,
              because the total number of recipients is not known at an
              earlier stage of the SMTP conversation.  Use at the RCPT stage
              will only reject the second etc.  recipient.
              The multi_recipient_bounce_reject_code parameter specifies the
              response code for rejected requests (default:  550).  This
              feature is available in Postfix 2.1 and later.

       reject_plaintext_session
              Reject the request when the connection is not encrypted. This
              restriction should not be used before the client has had a
              chance to negotiate encryption with the AUTH or STARTTLS
              commands.
              The plaintext_reject_code parameter specifies the response code
              for rejected requests (default:  450).  This feature is
              available in Postfix 2.3 and later.

       reject_unauth_pipelining
              Reject the request when the client sends SMTP commands ahead of
              time where it is not allowed, or when the client sends SMTP
              commands ahead of time without knowing that Postfix actually
              supports ESMTP command pipelining. This stops mail from bulk
              mail software that improperly uses ESMTP command pipelining in
              order to speed up deliveries.
              With Postfix 2.6 and later, the SMTP server sets a per-session
              flag whenever it detects illegal pipelining, including pipelined
              HELO or EHLO commands. The reject_unauth_pipelining feature
              simply tests whether the flag was set at any point in time
              during the session.
              With older Postfix versions, reject_unauth_pipelining checks the
              current status of the input read queue, and its usage is not
              recommended in contexts other than smtpd_data_restrictions.

       reject Reject the request. This restriction is useful at the end of a
              restriction list, to make the default policy explicit.  The
              reject_code configuration parameter specifies the response code
              for rejected requests (default: 554).

       sleep seconds
              Pause for the specified number of seconds and proceed with the
              next restriction in the list, if any. This may stop zombie mail
              when used as:
              /etc/postfix/main.cf:
                  smtpd_client_restrictions =
                      sleep 1, reject_unauth_pipelining
                  smtpd_delay_reject = no
              This feature is available in Postfix 2.3.

       warn_if_reject
              A safety net for testing. When "warn_if_reject" is placed before
              a reject-type restriction, access table query, or
              check_policy_service query, this logs a "reject_warning" message
              instead of rejecting a request (when a reject-type restriction
              fails due to a temporary error, this logs a "reject_warning"
              message for any implicit "defer_if_permit" actions that would
              normally prevent mail from being accepted by some later access
              restriction). This feature has no effect on defer_if_reject
              restrictions.

       Other restrictions that are valid in this context:

       ⊕      SMTP command specific restrictions that are described under the
              smtpd_helo_restrictions, smtpd_sender_restrictions or
              smtpd_recipient_restrictions parameters. When helo, sender or
              recipient restrictions are listed under
              smtpd_client_restrictions, they have effect only with
              "smtpd_delay_reject = yes", so that $smtpd_client_restrictions
              is evaluated at the time of the RCPT TO command.

       Example:

       smtpd_client_restrictions = permit_mynetworks, reject_unknown_client_hostname

smtpd_command_filter (default: empty)
       A mechanism to transform commands from remote SMTP clients.  This is a
       last-resort tool to work around client commands that break
       interoperability with the Postfix SMTP server.  Other uses involve
       fault injection to test Postfix's handling of invalid commands.

       Specify the name of a "type:table" lookup table. The search string is
       the SMTP command as received from the remote SMTP client, except that
       initial whitespace and the trailing <CR><LF> are removed.  The result
       value is executed by the Postfix SMTP server.

       There is no need to use smtpd_command_filter for the following cases:

       ⊕      Use "resolve_numeric_domain = yes" to accept "user@ipaddress".

       ⊕      Postfix already accepts the correct form "user@[ipaddress]". Use
              virtual_alias_maps or canonical_maps to translate these into
              domain names if necessary.

       ⊕      Use "strict_rfc821_envelopes = no" to accept "RCPT TO:<User Name
              <user@example.com>>". Postfix will ignore the "User Name" part
              and deliver to the <user@example.com> address.

       Examples of problems that can be solved with the smtpd_command_filter
       feature:

       /etc/postfix/main.cf:
           smtpd_command_filter = pcre:/etc/postfix/command_filter

       /etc/postfix/command_filter:
           # Work around clients that send malformed HELO commands.
           /^HELO\s*$/ HELO domain.invalid

           # Work around clients that send empty lines.
           /^\s*$/     NOOP

           # Work around clients that send RCPT TO:<'user@domain'>.
           # WARNING: do not lose the parameters that follow the address.
           /^(RCPT\s+TO:\s*<)'([^[:space:]]+)'(>.*)/     $1$2$3

           # Append XVERP to MAIL FROM commands to request VERP-style delivery.
           # See VERP_README for more information on how to use Postfix VERP.
           /^(MAIL\s+FROM:\s*<listname@example\.com>.*)/   $1 XVERP

           # Bounce-never mail sink. Use notify_classes=bounce,resource,software
           # to send bounced mail to the postmaster (with message body removed).
           /^(RCPT\s+TO:\s*<.*>.*)\s+NOTIFY=\S+(.*)/     $1 NOTIFY=NEVER$2
           /^(RCPT\s+TO:.*)/                             $1 NOTIFY=NEVER

       This feature is available in Postfix 2.7.

smtpd_data_restrictions (default: empty)
       Optional access restrictions that the Postfix SMTP server applies in
       the context of the SMTP DATA command.  See SMTPD_ACCESS_README, section
       "Delayed evaluation of SMTP access restriction lists" for a discussion
       of evaluation context and time.

       This feature is available in Postfix 2.0 and later.

       Specify a list of restrictions, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace.
       Restrictions are applied in the order as specified; the first
       restriction that matches wins.

       The following restrictions are valid in this context:

       ⊕      Generic restrictions that can be used in any SMTP command
              context, described under smtpd_client_restrictions.

       ⊕      SMTP command specific restrictions described under
              smtpd_client_restrictions, smtpd_helo_restrictions,
              smtpd_sender_restrictions or smtpd_recipient_restrictions.

       ⊕      However, no recipient information is available in the case of
              multi-recipient mail. Acting on only one recipient would be
              misleading, because any decision will affect all recipients
              equally. Acting on all recipients would require a possibly very
              large amount of memory, and would also be misleading for the
              reasons mentioned before.

       Examples:

       smtpd_data_restrictions = reject_unauth_pipelining
       smtpd_data_restrictions = reject_multi_recipient_bounce

smtpd_delay_open_until_valid_rcpt (default: yes)
       Postpone the start of an SMTP mail transaction until a valid RCPT TO
       command is received. Specify "no" to create a mail transaction as soon
       as the Postfix SMTP server receives a valid MAIL FROM command.

       With sites that reject lots of mail, the default setting reduces the
       use of disk, CPU and memory resources. The downside is that rejected
       recipients are logged with NOQUEUE instead of a mail transaction ID.
       This complicates the logfile analysis of multi-recipient mail.

       This feature is available in Postfix 2.3 and later.

smtpd_delay_reject (default: yes)
       Wait until the RCPT TO command before evaluating
       $smtpd_client_restrictions, $smtpd_helo_restrictions and
       $smtpd_sender_restrictions, or wait until the ETRN command before
       evaluating $smtpd_client_restrictions and $smtpd_helo_restrictions.

       This feature is turned on by default because some clients apparently
       mis-behave when the Postfix SMTP server rejects commands before RCPT
       TO.

       The default setting has one major benefit: it allows Postfix to log
       recipient address information when rejecting a client name/address or
       sender address, so that it is possible to find out whose mail is being
       rejected.

smtpd_discard_ehlo_keyword_address_maps (default: empty)
       Lookup tables, indexed by the remote SMTP client address, with case
       insensitive lists of EHLO keywords (pipelining, starttls, auth, etc.)
       that the Postfix SMTP server will not send in the EHLO response to a
       remote SMTP client. See smtpd_discard_ehlo_keywords for details.  The
       tables are not searched by hostname for robustness reasons.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       This feature is available in Postfix 2.2 and later.

smtpd_discard_ehlo_keywords (default: empty)
       A case insensitive list of EHLO keywords (pipelining, starttls, auth,
       etc.) that the Postfix SMTP server will not send in the EHLO response
       to a remote SMTP client.

       This feature is available in Postfix 2.2 and later.

       Notes:

       ⊕      Specify the silent-discard pseudo keyword to prevent this action
              from being logged.

       ⊕      Use the smtpd_discard_ehlo_keyword_address_maps feature to
              discard EHLO keywords selectively.

smtpd_dns_reply_filter (default: empty)
       Optional filter for Postfix SMTP server DNS lookup results.  See
       smtp_dns_reply_filter for details including an example.

       This feature is available in Postfix 3.0 and later.

smtpd_end_of_data_restrictions (default: empty)
       Optional access restrictions that the Postfix SMTP server applies in
       the context of the SMTP END-OF-DATA command.  See SMTPD_ACCESS_README,
       section "Delayed evaluation of SMTP access restriction lists" for a
       discussion of evaluation context and time.

       This feature is available in Postfix 2.2 and later.

       See smtpd_data_restrictions for details and limitations.

smtpd_enforce_tls (default: no)
       Mandatory TLS: announce STARTTLS support to remote SMTP clients, and
       require that clients use TLS encryption.  According to RFC 2487 this
       MUST NOT be applied in case of a publicly-referenced SMTP server.  This
       option is therefore off by default.

       Note 1: "smtpd_enforce_tls = yes" implies "smtpd_tls_auth_only = yes".

       Note 2: when invoked via "sendmail -bs", Postfix will never offer
       STARTTLS due to insufficient privileges to access the server private
       key. This is intended behavior.

       This feature is available in Postfix 2.2 and later. With Postfix 2.3
       and later use smtpd_tls_security_level instead.

smtpd_error_sleep_time (default: 1s)
       With Postfix version 2.1 and later: the SMTP server response delay
       after a client has made more than $smtpd_soft_error_limit errors, and
       fewer than $smtpd_hard_error_limit errors, without delivering mail.

       With Postfix version 2.0 and earlier: the SMTP server delay before
       sending a reject (4xx or 5xx) response, when the client has made fewer
       than $smtpd_soft_error_limit errors without delivering mail. When the
       client has made $smtpd_soft_error_limit or more errors, delay all
       responses with the larger of (number of errors) seconds or
       $smtpd_error_sleep_time.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtpd_etrn_restrictions (default: empty)
       Optional restrictions that the Postfix SMTP server applies in the
       context of a client ETRN command.  See SMTPD_ACCESS_README, section
       "Delayed evaluation of SMTP access restriction lists" for a discussion
       of evaluation context and time.

       The Postfix ETRN implementation accepts only destinations that are
       eligible for the Postfix "fast flush" service. See the ETRN_README file
       for details.

       Specify a list of restrictions, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace.
       Restrictions are applied in the order as specified; the first
       restriction that matches wins.

       The following restrictions are specific to the domain name information
       received with the ETRN command.

       check_etrn_access type:table
              Search the specified access database for the ETRN domain name or
              its parent domains. See the access(5) manual page for details.

       Other restrictions that are valid in this context:

       ⊕      Generic restrictions that can be used in any SMTP command
              context, described under smtpd_client_restrictions.

       ⊕      SMTP command specific restrictions described under
              smtpd_client_restrictions and smtpd_helo_restrictions.

       Example:

       smtpd_etrn_restrictions = permit_mynetworks, reject

smtpd_expansion_filter (default: see postconf -d output)
       What characters are allowed in $name expansions of RBL reply templates.
       Characters not in the allowed set are replaced by "_".  Use C like
       escapes to specify special characters such as whitespace.

       The smtpd_expansion_filter value is not subject to Postfix
       configuration parameter $name expansion.

       This feature is available in Postfix 2.0 and later.

smtpd_forbidden_commands (default: CONNECT GET POST regexp:{{/^[^A-Z]/
       Bogus}})
       List of commands that cause the Postfix SMTP server to immediately
       terminate the session with a 221 code. This can be used to disconnect
       clients that obviously attempt to abuse the system. In addition to the
       commands listed in this parameter, commands that follow the "Label:"
       format of message headers will also cause a disconnect. With Postfix
       versions 3.6 and earlier, the default value is "CONNECT GET POST".

       This feature is available in Postfix 2.2 and later.

       Support for inline regular expressions was added in Postfix version
       3.7. See regexp_table(5) for a description of the syntax and features.

smtpd_hard_error_limit (default: normal: 20, overload: 1)
       The maximal number of errors a remote SMTP client is allowed to make
       without delivering mail. The Postfix SMTP server disconnects when the
       limit is reached. Normally the default limit is 20, but it changes
       under overload to just 1. With Postfix 2.5 and earlier, the SMTP server
       always allows up to 20 errors by default.  Valid values are greater
       than zero.

smtpd_helo_required (default: no)
       Require that a remote SMTP client introduces itself with the HELO or
       EHLO command before sending the MAIL command or other commands that
       require EHLO negotiation.

       Example:

       smtpd_helo_required = yes

smtpd_helo_restrictions (default: empty)
       Optional restrictions that the Postfix SMTP server applies in the
       context of a client HELO command.  See SMTPD_ACCESS_README, section
       "Delayed evaluation of SMTP access restriction lists" for a discussion
       of evaluation context and time.

       The default is to permit everything.

       Note: specify "smtpd_helo_required = yes" to fully enforce this
       restriction (without "smtpd_helo_required = yes", a client can simply
       skip smtpd_helo_restrictions by not sending HELO or EHLO).

       Specify a list of restrictions, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace.
       Restrictions are applied in the order as specified; the first
       restriction that matches wins.

       The following restrictions are specific to the hostname information
       received with the HELO or EHLO command.

       check_helo_access type:table
              Search the specified access(5) database for the HELO or EHLO
              hostname or parent domains, and execute the corresponding
              action.  Note: specify "smtpd_helo_required = yes" to fully
              enforce this restriction (without "smtpd_helo_required = yes", a
              client can simply skip check_helo_access by not sending HELO or
              EHLO).

       check_helo_a_access type:table
              Search the specified access(5) database for the IP addresses for
              the HELO or EHLO hostname, and execute the corresponding action.
              Note 1: a result of "OK" is not allowed for safety reasons.
              Instead, use DUNNO in order to exclude specific hosts from
              denylists.  Note 2: specify "smtpd_helo_required = yes" to fully
              enforce this restriction (without "smtpd_helo_required = yes", a
              client can simply skip check_helo_a_access by not sending HELO
              or EHLO).  This feature is available in Postfix 3.0 and later.

       check_helo_mx_access type:table
              Search the specified access(5) database for the MX hosts for the
              HELO or EHLO hostname, and execute the corresponding action.  If
              no MX record is found, look up A or AAAA records, just like the
              Postfix SMTP client would.  Note 1: a result of "OK" is not
              allowed for safety reasons. Instead, use DUNNO in order to
              exclude specific hosts from denylists.  Note 2: specify
              "smtpd_helo_required = yes" to fully enforce this restriction
              (without "smtpd_helo_required = yes", a client can simply skip
              check_helo_mx_access by not sending HELO or EHLO).  This feature
              is available in Postfix 2.1 and later.

       check_helo_ns_access type:table
              Search the specified access(5) database for the DNS servers for
              the HELO or EHLO hostname, and execute the corresponding action.
              Note 1: a result of "OK" is not allowed for safety reasons.
              Instead, use DUNNO in order to exclude specific hosts from
              denylists.  Note 2: specify "smtpd_helo_required = yes" to fully
              enforce this restriction (without "smtpd_helo_required = yes", a
              client can simply skip check_helo_ns_access by not sending HELO
              or EHLO). This feature is available in Postfix 2.1 and later.

       reject_invalid_helo_hostname (with Postfix < 2.3:
       reject_invalid_hostname)
              Reject the request when the HELO or EHLO hostname is malformed.
              Note: specify "smtpd_helo_required = yes" to fully enforce this
              restriction (without "smtpd_helo_required = yes", a client can
              simply skip reject_invalid_helo_hostname by not sending HELO or
              EHLO).
              The invalid_hostname_reject_code specifies the response code for
              rejected requests (default: 501).

       reject_non_fqdn_helo_hostname (with Postfix < 2.3:
       reject_non_fqdn_hostname)
              Reject the request when the HELO or EHLO hostname is not in
              fully-qualified domain or address literal form, as required by
              the RFC. Note: specify "smtpd_helo_required = yes" to fully
              enforce this restriction (without "smtpd_helo_required = yes", a
              client can simply skip reject_non_fqdn_helo_hostname by not
              sending HELO or EHLO).
              The non_fqdn_reject_code parameter specifies the response code
              for rejected requests (default: 504).

       reject_rhsbl_helo rbl_domain=d.d.d.d
              Reject the request when the HELO or EHLO hostname is listed with
              the A record "d.d.d.d" under rbl_domain (Postfix version 2.1 and
              later only).  Each "d" is a number, or a pattern inside "[]"
              that contains one or more ";"-separated numbers or
              number..number ranges (Postfix version 2.8 and later).  If no
              "=d.d.d.d" is specified, reject the request when the HELO or
              EHLO hostname is listed with any A record under rbl_domain. See
              the reject_rbl_client description for additional RBL related
              configuration parameters.  Note: specify "smtpd_helo_required =
              yes" to fully enforce this restriction (without
              "smtpd_helo_required = yes", a client can simply skip
              reject_rhsbl_helo by not sending HELO or EHLO). This feature is
              available in Postfix 2.0 and later.

       reject_unknown_helo_hostname (with Postfix < 2.3:
       reject_unknown_hostname)
              Reject the request when the HELO or EHLO hostname has no DNS A
              or MX record.
              The reply is specified with the unknown_hostname_reject_code
              parameter (default: 450) or
              unknown_helo_hostname_tempfail_action (default:
              defer_if_permit).  See the respective parameter descriptions for
              details.
              Note: specify "smtpd_helo_required = yes" to fully enforce this
              restriction (without "smtpd_helo_required = yes", a client can
              simply skip reject_unknown_helo_hostname by not sending HELO or
              EHLO).

       Other restrictions that are valid in this context:

       ⊕      Generic restrictions that can be used in any SMTP command
              context, described under smtpd_client_restrictions.

       ⊕      Client hostname or network address specific restrictions
              described under smtpd_client_restrictions.

       ⊕      SMTP command specific restrictions described under
              smtpd_sender_restrictions or smtpd_recipient_restrictions.  When
              sender or recipient restrictions are listed under
              smtpd_helo_restrictions, they have effect only with
              "smtpd_delay_reject = yes", so that $smtpd_helo_restrictions is
              evaluated at the time of the RCPT TO command.

       Examples:

       smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname
       smtpd_helo_restrictions = permit_mynetworks, reject_unknown_helo_hostname

smtpd_history_flush_threshold (default: 100)
       The maximal number of lines in the Postfix SMTP server command history
       before it is flushed upon receipt of EHLO, RSET, or end of DATA.

smtpd_junk_command_limit (default: normal: 100, overload: 1)
       The number of junk commands (NOOP, VRFY, ETRN or RSET) that a remote
       SMTP client can send before the Postfix SMTP server starts to increment
       the error counter with each junk command.  The junk command count is
       reset after mail is delivered.  See also the smtpd_error_sleep_time and
       smtpd_soft_error_limit configuration parameters.  Normally the default
       limit is 100, but it changes under overload to just 1. With Postfix 2.5
       and earlier, the SMTP server always allows up to 100 junk commands by
       default.

smtpd_log_access_permit_actions (default: empty)
       Enable logging of the named "permit" actions in SMTP server access
       lists (by default, the SMTP server logs "reject" actions but not
       "permit" actions).  This feature does not affect conditional actions
       such as "defer_if_permit".

       Specify a list of "permit" action names, "/file/name" or "type:table"
       patterns, separated by commas and/or whitespace. The list is matched
       left to right, and the search stops on the first match. A "/file/name"
       pattern is replaced by its contents; a "type:table" lookup table is
       matched when a name matches a lookup key (the lookup result is
       ignored).  Continue long lines by starting the next line with
       whitespace. Specify "!pattern" to exclude a name from the list.

       Examples:

       /etc/postfix/main.cf:
           # Log all "permit" actions.
           smtpd_log_access_permit_actions = static:all

       /etc/postfix/main.cf:
           # Log "permit_dnswl_client" only.
           smtpd_log_access_permit_actions = permit_dnswl_client

       This feature is available in Postfix 2.10 and later.

smtpd_milter_maps (default: empty)
       Lookup tables with Milter settings per remote SMTP client IP address.
       The lookup result overrides the smtpd_milters setting, and has the same
       syntax.

       Note: lookup tables cannot return empty responses. Specify a lookup
       result of DISABLE (case does not matter) to indicate that Milter
       support should be disabled.

       Example to disable Milters for local clients:

       /etc/postfix/main.cf:
           smtpd_milter_maps = cidr:/etc/postfix/smtpd_milter_map
           smtpd_milters = inet:host:port, { inet:host:port, ... }, ...

       /etc/postfix/smtpd_milter_map:
           # Disable Milters for local clients.
           127.0.0.0/8    DISABLE
           192.168.0.0/16 DISABLE
           ::/64          DISABLE
           2001:db8::/32  DISABLE

       This feature is available in Postfix 3.2 and later.

smtpd_milters (default: empty)
       A list of Milter (mail filter) applications for new mail that arrives
       via the Postfix smtpd(8) server. Specify space or comma as separator.
       See the MILTER_README document for details.

       This feature is available in Postfix 2.3 and later.

smtpd_min_data_rate (default: 500)
       The minimum plaintext data transfer rate in bytes/second for DATA and
       BDAT requests, when deadlines are enabled with
       smtpd_per_request_deadline. After a read operation transfers N
       plaintext message bytes (possibly after TLS decryption), and after the
       DATA or BDAT request deadline is decremented by the elapsed time of
       that read operation, the DATA or BDAT request deadline is incremented
       by N/smtpd_min_data_rate seconds. However, the deadline will never be
       incremented beyond the time limit specified with smtpd_timeout.

       This feature is available in Postfix 3.7 and later.

smtpd_noop_commands (default: empty)
       List of commands that the Postfix SMTP server replies to with "250 Ok",
       without doing any syntax checks and without changing state.  This list
       overrides any commands built into the Postfix SMTP server.

smtpd_null_access_lookup_key (default: <>)
       The lookup key to be used in SMTP access(5) tables instead of the null
       sender address.

smtpd_peername_lookup (default: yes)
       Attempt to look up the remote SMTP client hostname, and verify that the
       name matches the client IP address. A client name is set to "unknown"
       when it cannot be looked up or verified, or when name lookup is
       disabled.  Turning off name lookup reduces delays due to DNS lookup and
       increases the maximal inbound delivery rate.

       This feature is available in Postfix 2.3 and later.

smtpd_per_record_deadline (default: normal: no, overload: yes)
       Change the behavior of the smtpd_timeout and smtpd_starttls_timeout
       time limits, from a time limit per read or write system call, to a time
       limit to send or receive a complete record (an SMTP command line, SMTP
       response line, SMTP message content line, or TLS protocol message).
       This limits the impact from hostile peers that trickle data one byte at
       a time.

       Note: when per-record deadlines are enabled, a short timeout may cause
       problems with TLS over very slow network connections.  The reasons are
       that a TLS protocol message can be up to 16 kbytes long (with TLSv1),
       and that an entire TLS protocol message must be sent or received within
       the per-record deadline.

       This feature is available in Postfix 2.9-3.6. With older Postfix
       releases, the behavior is as if this parameter is set to "no". Postfix
       3.7 and later use smtpd_per_request_deadline.

smtpd_per_request_deadline (default: normal: no, overload: yes)
       Change the behavior of the smtpd_timeout and smtpd_starttls_timeout
       time limits, from a time limit per plaintext or TLS read or write call,
       to a combined time limit for receiving a complete SMTP request and for
       sending a complete SMTP response. The deadline limits only the time
       spent waiting for plaintext or TLS read or write calls, not time spent
       elsewhere. The per-request deadline limits the impact from hostile
       peers that trickle data one byte at a time.

       See smtpd_min_data_rate for how the per-request deadline is managed
       during the DATA and BDAT phase.

       Note: when per-request deadlines are enabled, a short time limit may
       cause problems with TLS over very slow network connections. The reason
       is that a TLS protocol message can be up to 16 kbytes long (with
       TLSv1), and that an entire TLS protocol message must be transferred
       within the per-request deadline.

       This feature is available in Postfix 3.7 and later. A weaker feature,
       called smtpd_per_record_deadline, is available with Postfix 2.9-3.6.
       With older Postfix releases, the behavior is as if this parameter is
       set to "no".

       This feature is available in Postfix 3.7 and later.

smtpd_policy_service_default_action (default: 451 4.3.5 Server configuration
       problem)
       The default action when an SMTPD policy service request fails.  Specify
       "DUNNO" to behave as if the failed  SMTPD policy service request was
       not sent, and to continue processing other access restrictions, if any.

       Limitations:

       ⊕      This parameter may specify any value that would be a valid SMTPD
              policy server response (or access(5) map lookup result).  An
              access(5) map or policy server in this parameter value may need
              to be declared in advance with a restriction_class setting.

       ⊕      If the specified action invokes another check_policy_service
              request, that request will have the built-in default action.

       This feature is available in Postfix 3.0 and later.

smtpd_policy_service_max_idle (default: 300s)
       The time after which an idle SMTPD policy service connection is closed.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

smtpd_policy_service_max_ttl (default: 1000s)
       The time after which an active SMTPD policy service connection is
       closed.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

smtpd_policy_service_policy_context (default: empty)
       Optional information that the Postfix SMTP server specifies in the
       "policy_context" attribute of a policy service request (originally, to
       share the same service endpoint among multiple check_policy_service
       clients).

       This feature is available in Postfix 3.1 and later.

smtpd_policy_service_request_limit (default: 0)
       The maximal number of requests per SMTPD policy service connection, or
       zero (no limit). Once a connection reaches this limit, the connection
       is closed and the next request will be sent over a new connection. This
       is a workaround to avoid error-recovery delays with policy servers that
       cannot maintain a persistent connection.

       This feature is available in Postfix 3.0 and later.

smtpd_policy_service_retry_delay (default: 1s)
       The delay between attempts to resend a failed SMTPD policy service
       request. Specify a value greater than zero.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 3.0 and later.

smtpd_policy_service_timeout (default: 100s)
       The time limit for connecting to, writing to, or receiving from a
       delegated SMTPD policy server.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

smtpd_policy_service_try_limit (default: 2)
       The maximal number of attempts to send an SMTPD policy service request
       before giving up. Specify a value greater than zero.

       This feature is available in Postfix 3.0 and later.

smtpd_proxy_ehlo (default: $myhostname)
       How the Postfix SMTP server announces itself to the proxy filter.  By
       default, the Postfix hostname is used.

       This feature is available in Postfix 2.1 and later.

smtpd_proxy_filter (default: empty)
       The hostname and TCP port of the mail filtering proxy server.  The
       proxy receives all mail from the Postfix SMTP server, and is supposed
       to give the result to another Postfix SMTP server process.

       Specify "host:port" or "inet:host:port" for a TCP endpoint, or
       "unix:pathname" for a UNIX-domain endpoint. The host can be specified
       as an IP address or as a symbolic name; no MX lookups are done.  When
       no "host" or "host:" is specified, the local machine is assumed.
       Pathname interpretation is relative to the Postfix queue directory.

       This feature is available in Postfix 2.1 and later.

       The "inet:" and "unix:" prefixes are available in Postfix 2.3 and
       later.

smtpd_proxy_options (default: empty)
       List of options that control how the Postfix SMTP server communicates
       with a before-queue content filter. Specify zero or more of the
       following, separated by comma or whitespace.

       speed_adjust
              Do not connect to a before-queue content filter until an entire
              message has been received. This reduces the number of
              simultaneous before-queue content filter processes.

       NOTE 1: A filter must not selectively reject recipients of a
       multi-recipient message.  Rejecting all recipients is OK, as is
       accepting all recipients.

       NOTE 2: This feature increases the minimum amount of free queue space
       by $message_size_limit. The extra space is needed to save the message
       to a temporary file.

       This feature is available in Postfix 2.7 and later.

smtpd_proxy_timeout (default: 100s)
       The time limit for connecting to a proxy filter and for sending or
       receiving information.  When a connection fails the client gets a
       generic error message while more detailed information is logged to the
       maillog file.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.1 and later.

smtpd_recipient_limit (default: 1000)
       The maximal number of recipients that the Postfix SMTP server accepts
       per message delivery request.

smtpd_recipient_overshoot_limit (default: 1000)
       The number of recipients that a remote SMTP client can send in excess
       of the limit specified with $smtpd_recipient_limit, before the Postfix
       SMTP server increments the per-session error count for each excess
       recipient.

smtpd_recipient_restrictions (default: see postconf -d output)
       Optional restrictions that the Postfix SMTP server applies in the
       context of a client RCPT TO command, after smtpd_relay_restrictions.
       See SMTPD_ACCESS_README, section "Delayed evaluation of SMTP access
       restriction lists" for a discussion of evaluation context and time.

       With Postfix versions before 2.10, the rules for relay permission and
       spam blocking were combined under smtpd_recipient_restrictions,
       resulting in error-prone configuration.  As of Postfix 2.10, relay
       permission rules are preferably implemented with
       smtpd_relay_restrictions, so that a permissive spam blocking policy
       under smtpd_recipient_restrictions will no longer result in a
       permissive mail relay policy.

       For backwards compatibility, sites that migrate from Postfix versions
       before 2.10 can set smtpd_relay_restrictions to the empty value, and
       use smtpd_recipient_restrictions exactly as before.

       IMPORTANT: Either the smtpd_relay_restrictions or the
       smtpd_recipient_restrictions parameter must specify at least one of the
       following restrictions. Otherwise Postfix will refuse to receive mail:

           reject, reject_unauth_destination

           defer, defer_if_permit, defer_unauth_destination

       Specify a list of restrictions, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace.
       Restrictions are applied in the order as specified; the first
       restriction that matches wins.

       The following restrictions are specific to the recipient address that
       is received with the RCPT TO command.

       check_recipient_access type:table
              Search the specified access(5) database for the resolved RCPT TO
              address, domain, parent domains, or localpart@, and execute the
              corresponding action.

       check_recipient_a_access type:table
              Search the specified access(5) database for the IP addresses for
              the RCPT TO domain, and execute the corresponding action.  Note:
              a result of "OK" is not allowed for safety reasons. Instead, use
              DUNNO in order to exclude specific hosts from denylists.  This
              feature is available in Postfix 3.0 and later.

       check_recipient_mx_access type:table
              Search the specified access(5) database for the MX hosts for the
              RCPT TO domain, and execute the corresponding action.  If no MX
              record is found, look up A or AAAA records, just like the
              Postfix SMTP client would. Note: a result of "OK" is not allowed
              for safety reasons. Instead, use DUNNO in order to exclude
              specific hosts from denylists.  This feature is available in
              Postfix 2.1 and later.

       check_recipient_ns_access type:table
              Search the specified access(5) database for the DNS servers for
              the RCPT TO domain, and execute the corresponding action.  Note:
              a result of "OK" is not allowed for safety reasons. Instead, use
              DUNNO in order to exclude specific hosts from denylists.  This
              feature is available in Postfix 2.1 and later.

       permit_auth_destination
              Permit the request when one of the following is true:

       ⊕      Postfix is a mail forwarder: the resolved RCPT TO domain matches
              $relay_domains or a subdomain thereof, and the address contains
              no sender-specified routing (user@elsewhere@domain),

       ⊕      Postfix is the final destination: the resolved RCPT TO domain
              matches $mydestination, $inet_interfaces, $proxy_interfaces,
              $virtual_alias_domains, or $virtual_mailbox_domains, and the
              address contains no sender-specified routing
              (user@elsewhere@domain).

       permit_mx_backup
              Permit the request when the local mail system is a backup MX for
              the RCPT TO domain, or when the domain is an authorized
              destination (see permit_auth_destination for definition).

       ⊕      Safety: permit_mx_backup does not accept addresses that have
              sender-specified routing information (example:
              user@elsewhere@domain).

       ⊕      Safety: permit_mx_backup can be vulnerable to mis-use when
              access is not restricted with permit_mx_backup_networks.

       ⊕      Safety: as of Postfix version 2.3, permit_mx_backup no longer
              accepts the address when the local mail system is a primary MX
              for the recipient domain.  Exception: permit_mx_backup accepts
              the address when it specifies an authorized destination (see
              permit_auth_destination for definition).

       ⊕      Limitation: mail may be rejected in case of a temporary DNS
              lookup problem with Postfix prior to version 2.0.

       reject_non_fqdn_recipient
              Reject the request when the RCPT TO address specifies a domain
              that is not in fully-qualified domain form, as required by the
              RFC.
              The non_fqdn_reject_code parameter specifies the response code
              for rejected requests (default: 504).

       reject_rhsbl_recipient rbl_domain=d.d.d.d
              Reject the request when the RCPT TO domain is listed with the A
              record "d.d.d.d" under rbl_domain (Postfix version 2.1 and later
              only).  Each "d" is a number, or a pattern inside "[]" that
              contains one or more ";"-separated numbers or number..number
              ranges (Postfix version 2.8 and later). If no "=d.d.d.d" is
              specified, reject the request when the RCPT TO domain is listed
              with any A record under rbl_domain.
              The maps_rbl_reject_code parameter specifies the response code
              for rejected requests (default: 554); the default_rbl_reply
              parameter specifies the default server reply; and the
              rbl_reply_maps parameter specifies tables with server replies
              indexed by rbl_domain.  This feature is available in Postfix
              version 2.0 and later.

       reject_unauth_destination
              Reject the request unless one of the following is true:

       ⊕      Postfix is a mail forwarder: the resolved RCPT TO domain matches
              $relay_domains or a subdomain thereof, and contains no
              sender-specified routing (user@elsewhere@domain),

       ⊕      Postfix is the final destination: the resolved RCPT TO domain
              matches $mydestination, $inet_interfaces, $proxy_interfaces,
              $virtual_alias_domains, or $virtual_mailbox_domains, and
              contains no sender-specified routing (user@elsewhere@domain).
              The relay_domains_reject_code parameter specifies the response
              code for rejected requests (default: 554).

       defer_unauth_destination
              Reject the same requests as reject_unauth_destination, with a
              non-permanent error code.  This feature is available in Postfix
              2.10 and later.

       reject_unknown_recipient_domain
              Reject the request when Postfix is not final destination for the
              recipient domain, and the RCPT TO domain has 1) no DNS MX and no
              DNS A record or 2) a malformed MX record such as a record with a
              zero-length MX hostname (Postfix version 2.3 and later).
              The reply is specified with the unknown_address_reject_code
              parameter (default: 450), unknown_address_tempfail_action
              (default: defer_if_permit), or 556 (nullmx, Postfix 3.0 and
              later). See the respective parameter descriptions for details.

       reject_unlisted_recipient (with Postfix version 2.0:
       check_recipient_maps)
              Reject the request when the RCPT TO address is not listed in the
              list of valid recipients for its domain class. See the
              smtpd_reject_unlisted_recipient parameter description for
              details.  This feature is available in Postfix 2.1 and later.

       reject_unverified_recipient
              Reject the request when mail to the RCPT TO address is known to
              bounce, or when the recipient address destination is not
              reachable.  Address verification information is managed by the
              verify(8) server; see the ADDRESS_VERIFICATION_README file for
              details.
              The unverified_recipient_reject_code parameter specifies the
              numerical response code when an address is known to bounce
              (default: 450, change it to 550 when you are confident that it
              is safe to do so).
              The unverified_recipient_defer_code parameter specifies the
              numerical response code when an address probe failed due to a
              temporary problem (default: 450).
              The unverified_recipient_tempfail_action parameter specifies the
              action after address probe failure due to a temporary problem
              (default: defer_if_permit).
              This feature breaks for aliased addresses with
              "enable_original_recipient = no" (Postfix <= 3.2).
              This feature is available in Postfix 2.1 and later.

       Other restrictions that are valid in this context:

       ⊕      Generic restrictions that can be used in any SMTP command
              context, described under smtpd_client_restrictions.

       ⊕      SMTP command specific restrictions described under
              smtpd_client_restrictions, smtpd_helo_restrictions and
              smtpd_sender_restrictions.

       Example:

       # The Postfix before 2.10 default mail relay policy. Later Postfix
       # versions implement this preferably with smtpd_relay_restrictions.
       smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination

smtpd_reject_footer (default: empty)
       Optional information that is appended after each Postfix SMTP server
       4XX or 5XX response.

       The following example uses "\c" at the start of the template (supported
       in Postfix 2.10 and later) to suppress the line break between the reply
       text and the footer text. With earlier Postfix versions, the footer
       text always begins on a new line, and the "\c" is output literally.

       /etc/postfix/main.cf:
           smtpd_reject_footer = \c. For assistance, call 800-555-0101.
            Please provide the following information in your problem report:
            time ($localtime), client ($client_address) and server
            ($server_name).

       Server response:

           550-5.5.1 <user@example> Recipient address rejected: User
           unknown. For assistance, call 800-555-0101. Please provide the
           following information in your problem report: time (Jan 4 15:42:00),
           client (192.168.1.248) and server (mail1.example.com).

       Note: the above text is meant to make it easier to find the Postfix
       logfile records for a failed SMTP session. The text itself is not
       logged to the Postfix SMTP server's maillog file.

       Be sure to keep the text as short as possible. Long text may be
       truncated before it is logged to the remote SMTP client's maillog file,
       or before it is returned to the sender in a delivery status
       notification.

       The template text is not subject to Postfix configuration parameter
       $name expansion. Instead, this feature supports a limited number of
       $name attributes in the footer text. These attributes are replaced with
       their current value for the SMTP session.

       Note: specify $$name in footer text that is looked up from regexp: or
       pcre:-based smtpd_reject_footer_maps, otherwise the Postfix server will
       not use the footer text and will log a warning instead.

       client_address
              The Client IP address that is logged in the maillog file.

       client_port
              The client TCP port that is logged in the maillog file.

       localtime
              The server local time (Mmm dd hh:mm:ss) that is logged in the
              maillog file.

       server_name
              The server's myhostname value.  This attribute is made available
              for sites with multiple MTAs (perhaps behind a load-balancer),
              where the server name can help the server support team to
              quickly find the right log files.

       Notes:

       ⊕      NOT SUPPORTED are other attributes such as sender, recipient, or
              main.cf parameters.

       ⊕      For safety reasons, text that does not match
              $smtpd_expansion_filter is censored.

       This feature supports the two-character sequence \n as a request for a
       line break in the footer text. Postfix automatically inserts after each
       line break the three-digit SMTP reply code (and optional enhanced
       status code) from the original Postfix reject message.

       To work around mail software that mis-handles multi-line replies,
       specify the two-character sequence \c at the start of the template.
       This suppresses the line break between the reply text and the footer
       text (Postfix 2.10 and later).

       This feature is available in Postfix 2.8 and later.

smtpd_reject_footer_maps (default: empty)
       Lookup tables, indexed by the complete Postfix SMTP server 4xx or 5xx
       response, with reject footer templates. See smtpd_reject_footer for
       details.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       This feature is available in Postfix 3.4 and later.

smtpd_reject_unlisted_recipient (default: yes)
       Request that the Postfix SMTP server rejects mail for unknown recipient
       addresses, even when no explicit reject_unlisted_recipient access
       restriction is specified. This prevents the Postfix queue from filling
       up with undeliverable MAILER-DAEMON messages.

       An address is always considered "known" when it matches a virtual(5)
       alias or a canonical(5) mapping.

       ⊕      The recipient domain matches $mydestination, $inet_interfaces or
              $proxy_interfaces, but the recipient is not listed in
              $local_recipient_maps, and $local_recipient_maps is not null.

       ⊕      The recipient domain matches $virtual_alias_domains but the
              recipient is not listed in $virtual_alias_maps.

       ⊕      The recipient domain matches $virtual_mailbox_domains but the
              recipient is not listed in $virtual_mailbox_maps, and
              $virtual_mailbox_maps is not null.

       ⊕      The recipient domain matches $relay_domains but the recipient is
              not listed in $relay_recipient_maps, and $relay_recipient_maps
              is not null.

       This feature is available in Postfix 2.1 and later.

smtpd_reject_unlisted_sender (default: no)
       Request that the Postfix SMTP server rejects mail from unknown sender
       addresses, even when no explicit reject_unlisted_sender access
       restriction is specified. This can slow down an explosion of forged
       mail from worms or viruses.

       An address is always considered "known" when it matches a virtual(5)
       alias or a canonical(5) mapping.

       ⊕      The sender domain matches $mydestination, $inet_interfaces or
              $proxy_interfaces, but the sender is not listed in
              $local_recipient_maps, and $local_recipient_maps is not null.

       ⊕      The sender domain matches $virtual_alias_domains but the sender
              is not listed in $virtual_alias_maps.

       ⊕      The sender domain matches $virtual_mailbox_domains but the
              sender is not listed in $virtual_mailbox_maps, and
              $virtual_mailbox_maps is not null.

       ⊕      The sender domain matches $relay_domains but the sender is not
              listed in $relay_recipient_maps, and $relay_recipient_maps is
              not null.

       This feature is available in Postfix 2.1 and later.

smtpd_relay_before_recipient_restrictions (default: see postconf -d output)
       Evaluate smtpd_relay_restrictions before smtpd_recipient_restrictions.
       Historically, smtpd_relay_restrictions was evaluated after
       smtpd_recipient_restrictions, contradicting documented behavior.

       Background: the smtpd_relay_restrictions feature is primarily designed
       to enforce a mail relaying policy, while smtpd_recipient_restrictions
       is primarily designed to enforce spam blocking policy. Both are
       evaluated while replying to the RCPT TO command, and both support the
       same features.

       This feature is available in Postfix 3.6 and later.

smtpd_relay_restrictions (default: permit_mynetworks,
       permit_sasl_authenticated, defer_unauth_destination)
       Access restrictions for mail relay control that the Postfix SMTP server
       applies in the context of the RCPT TO command, before
       smtpd_recipient_restrictions.  See SMTPD_ACCESS_README, section
       "Delayed evaluation of SMTP access restriction lists" for a discussion
       of evaluation context and time.

       With Postfix versions before 2.10, the rules for relay permission and
       spam blocking were combined under smtpd_recipient_restrictions,
       resulting in error-prone configuration.  As of Postfix 2.10, relay
       permission rules are preferably implemented with
       smtpd_relay_restrictions, so that a permissive spam blocking policy
       under smtpd_recipient_restrictions will no longer result in a
       permissive mail relay policy.

       For backwards compatibility, sites that migrate from Postfix versions
       before 2.10 can set smtpd_relay_restrictions to the empty value, and
       use smtpd_recipient_restrictions exactly as before.

       By default, the Postfix SMTP server accepts:

       ⊕      Mail from clients whose IP address matches $mynetworks, or:

       ⊕      Mail from clients who are SASL authenticated, or:

       ⊕      Mail to remote destinations that match $relay_domains, except
              for addresses that contain sender-specified routing
              (user@elsewhere@domain), or:

       ⊕      Mail to local destinations that match $inet_interfaces or
              $proxy_interfaces, $mydestination, $virtual_alias_domains, or
              $virtual_mailbox_domains.

       IMPORTANT: Either the smtpd_relay_restrictions or the
       smtpd_recipient_restrictions parameter must specify at least one of the
       following restrictions. Otherwise Postfix will refuse to receive mail:

           reject, reject_unauth_destination

           defer, defer_if_permit, defer_unauth_destination

       Specify a list of restrictions, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace.  The
       same restrictions are available as documented under
       smtpd_recipient_restrictions.

       This feature is available in Postix 2.10 and later.

smtpd_restriction_classes (default: empty)
       User-defined aliases for groups of access restrictions. The aliases can
       be specified in smtpd_recipient_restrictions etc., and on the
       right-hand side of a Postfix access(5) table.

       One major application is for implementing per-recipient UCE control.
       See the RESTRICTION_CLASS_README document for other examples.

smtpd_sasl_application_name (default: smtpd)
       The application name that the Postfix SMTP server uses for SASL server
       initialization. This controls the name of the SASL configuration file.
       The default value is smtpd, corresponding to a SASL configuration file
       named smtpd.conf.

       This feature is available in Postfix 2.1 and 2.2. With Postfix 2.3 it
       was renamed to smtpd_sasl_path.

smtpd_sasl_auth_enable (default: no)
       Enable SASL authentication in the Postfix SMTP server. By default, the
       Postfix SMTP server does not use authentication.

       If a remote SMTP client is authenticated, the permit_sasl_authenticated
       access restriction can be used to permit relay access, like this:

           # With Postfix 2.10 and later, the mail relay policy is
           # preferably specified under smtpd_relay_restrictions.
           smtpd_relay_restrictions =
               permit_mynetworks, permit_sasl_authenticated, ...

       # With Postfix before 2.10, the relay policy can be
       # specified only under smtpd_recipient_restrictions.
       smtpd_recipient_restrictions =
           permit_mynetworks, permit_sasl_authenticated, ...

       To reject all SMTP connections from unauthenticated clients, specify
       "smtpd_delay_reject = yes" (which is the default) and use:

           smtpd_client_restrictions = permit_sasl_authenticated, reject

       See the SASL_README file for SASL configuration and operation details.

smtpd_sasl_authenticated_header (default: no)
       Report the SASL authenticated user name in the smtpd(8) Received
       message header.

       This feature is available in Postfix 2.3 and later.

smtpd_sasl_exceptions_networks (default: empty)
       What remote SMTP clients the Postfix SMTP server will not offer AUTH
       support to.

       Some clients (Netscape 4 at least) have a bug that causes them to
       require a login and password whenever AUTH is offered, whether it's
       necessary or not. To work around this, specify, for example,
       $mynetworks to prevent Postfix from offering AUTH to local clients.

       Specify a list of network/netmask patterns, separated by commas and/or
       whitespace. The mask specifies the number of bits in the network part
       of a host address. You can also specify "/file/name" or "type:table"
       patterns.  A "/file/name" pattern is replaced by its contents; a
       "type:table" lookup table is matched when a table entry matches a
       lookup string (the lookup result is ignored).  Continue long lines by
       starting the next line with whitespace. Specify "!pattern" to exclude
       an address or network block from the list.  The form "!/file/name" is
       supported only in Postfix version 2.4 and later.

       Note: IP version 6 address information must be specified inside [] in
       the smtpd_sasl_exceptions_networks value, and in files specified with
       "/file/name".  IP version 6 addresses contain the ":" character, and
       would otherwise be confused with a "type:table" pattern.

       Example:

       smtpd_sasl_exceptions_networks = $mynetworks

       This feature is available in Postfix 2.1 and later.

smtpd_sasl_local_domain (default: empty)
       The name of the Postfix SMTP server's local SASL authentication realm.

       By default, the local authentication realm name is the null string.

       Examples:

       smtpd_sasl_local_domain = $mydomain
       smtpd_sasl_local_domain = $myhostname

smtpd_sasl_mechanism_filter (default: !external, static:rest)
       If non-empty, a filter for the SASL mechanism names that the Postfix
       SMTP server will announce in the EHLO response. By default, the Postfix
       SMTP server will not announce the EXTERNAL mechanism, because Postfix
       support for that is not implemented.

       Specify mechanism names, "/file/name" patterns, or "type:table" lookup
       tables, separated by comma or whitespace. The right-hand side result
       from "type:table" lookups is ignored. Specify "!pattern" to exclude a
       mechanism name from the list.

       Examples:

       smtpd_sasl_mechanism_filter = !external, !gssapi, static:rest
       smtpd_sasl_mechanism_filter = login, plain
       smtpd_sasl_mechanism_filter = /etc/postfix/smtpd_mechs

       This feature is available in Postfix 3.6 and later.

smtpd_sasl_path (default: smtpd)
       Implementation-specific information that the Postfix SMTP server passes
       through to the SASL plug-in implementation that is selected with
       smtpd_sasl_type.  Typically this specifies the name of a configuration
       file or rendezvous point.

       This feature is available in Postfix 2.3 and later. In earlier releases
       it was called smtpd_sasl_application_name.

smtpd_sasl_response_limit (default: 12288)
       The maximum length of a SASL client's response to a server challenge.
       When the client's "initial response" is longer than the normal limit
       for SMTP commands, the client must omit its initial response, and wait
       for an empty server challenge; it can then send what would have been
       its "initial response" as a response to the empty server challenge.
       RFC4954 requires the server to accept client responses up to at least
       12288 octets of base64-encoded text.  The default value is therefore
       also the minimum value accepted for this parameter.

       This feature is available in Postfix 3.4 and later. Prior versions use
       "line_length_limit", which may need to be raised to accommodate larger
       client responses, as may be needed with GSSAPI authentication of
       Windows AD users who are members of many groups.

smtpd_sasl_security_options (default: noanonymous)
       Postfix SMTP server SASL security options; as of Postfix 2.3 the list
       of available features depends on the SASL server implementation that is
       selected with smtpd_sasl_type.

       The following security features are defined for the cyrus server SASL
       implementation:

       Restrict what authentication mechanisms the Postfix SMTP server will
       offer to the client.  The list of available authentication mechanisms
       is system dependent.

       Specify zero or more of the following:

       noplaintext
              Disallow methods that use plaintext passwords.

       noactive
              Disallow methods subject to active (non-dictionary) attack.

       nodictionary
              Disallow methods subject to passive (dictionary) attack.

       noanonymous
              Disallow methods that allow anonymous authentication.

       forward_secrecy
              Only allow methods that support forward secrecy (Dovecot only).

       mutual_auth
              Only allow methods that provide mutual authentication (not
              available with Cyrus SASL version 1).

       By default, the Postfix SMTP server accepts plaintext passwords but not
       anonymous logins.

       Warning: it appears that clients try authentication methods in the
       order as advertised by the server (e.g., PLAIN ANONYMOUS CRAM-MD5)
       which means that if you disable plaintext passwords, clients will log
       in anonymously, even when they should be able to use CRAM-MD5.  So, if
       you disable plaintext logins, disable anonymous logins too.  Postfix
       treats anonymous login as no authentication.

       Example:

       smtpd_sasl_security_options = noanonymous, noplaintext

smtpd_sasl_service (default: smtp)
       The service name that is passed to the SASL plug-in that is selected
       with smtpd_sasl_type and smtpd_sasl_path.

       This feature is available in Postfix 2.11 and later. Prior versions
       behave as if "smtp" is specified.

smtpd_sasl_tls_security_options (default: $smtpd_sasl_security_options)
       The SASL authentication security options that the Postfix SMTP server
       uses for TLS encrypted SMTP sessions.

       This feature is available in Postfix 2.2 and later.

smtpd_sasl_type (default: cyrus)
       The SASL plug-in type that the Postfix SMTP server should use for
       authentication. The available types are listed with the "postconf -a"
       command.

       This feature is available in Postfix 2.3 and later.

smtpd_sender_login_maps (default: empty)
       Optional lookup table with the SASL login names that own the sender
       (MAIL FROM) addresses.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.  With lookups from indexed files such as DB or DBM, or from
       networked tables such as NIS, LDAP or SQL, the following search
       operations are done with a sender address of user@domain:

       1) user@domain
              This table lookup is always done and has the highest precedence.

       2) user
              This table lookup is done only when the domain part of the
              sender address matches $myorigin, $mydestination,
              $inet_interfaces or $proxy_interfaces.

       3) @domain
              This table lookup is done last and has the lowest precedence.

       In all cases the result of table lookup must be either "not found" or a
       list of SASL login names separated by comma and/or whitespace.

smtpd_sender_restrictions (default: empty)
       Optional restrictions that the Postfix SMTP server applies in the
       context of a client MAIL FROM command.  See SMTPD_ACCESS_README,
       section "Delayed evaluation of SMTP access restriction lists" for a
       discussion of evaluation context and time.

       The default is to permit everything.

       Specify a list of restrictions, separated by commas and/or whitespace.
       Continue long lines by starting the next line with whitespace.
       Restrictions are applied in the order as specified; the first
       restriction that matches wins.

       The following restrictions are specific to the sender address received
       with the MAIL FROM command.

       check_sender_access type:table
              Search the specified access(5) database for the MAIL FROM
              address, domain, parent domains, or localpart@, and execute the
              corresponding action.

       check_sender_a_access type:table
              Search the specified access(5) database for the IP addresses for
              the MAIL FROM domain, and execute the corresponding action.
              Note: a result of "OK" is not allowed for safety reasons.
              Instead, use DUNNO in order to exclude specific hosts from
              denylists.  This feature is available in Postfix 3.0 and later.

       check_sender_mx_access type:table
              Search the specified access(5) database for the MX hosts for the
              MAIL FROM domain, and execute the corresponding action.  If no
              MX record is found, look up A or AAAA records, just like the
              Postfix SMTP client would. Note: a result of "OK" is not allowed
              for safety reasons. Instead, use DUNNO in order to exclude
              specific hosts from denylists.  This feature is available in
              Postfix 2.1 and later.

       check_sender_ns_access type:table
              Search the specified access(5) database for the DNS servers for
              the MAIL FROM domain, and execute the corresponding action.
              Note: a result of "OK" is not allowed for safety reasons.
              Instead, use DUNNO in order to exclude specific hosts from
              denylists.  This feature is available in Postfix 2.1 and later.

       reject_authenticated_sender_login_mismatch
              Enforces the reject_sender_login_mismatch restriction for
              authenticated clients only. This feature is available in Postfix
              version 2.1 and later.

       reject_known_sender_login_mismatch
              Apply the reject_sender_login_mismatch restriction only to MAIL
              FROM addresses that are known in $smtpd_sender_login_maps.  This
              feature is available in Postfix version 2.11 and later.

       reject_non_fqdn_sender
              Reject the request when the MAIL FROM address specifies a domain
              that is not in fully-qualified domain form as required by the
              RFC.
              The non_fqdn_reject_code parameter specifies the response code
              for rejected requests (default: 504).

       reject_rhsbl_sender rbl_domain=d.d.d.d
              Reject the request when the MAIL FROM domain is listed with the
              A record "d.d.d.d" under rbl_domain (Postfix version 2.1 and
              later only).  Each "d" is a number, or a pattern inside "[]"
              that contains one or more ";"-separated numbers or
              number..number ranges (Postfix version 2.8 and later). If no
              "=d.d.d.d" is specified, reject the request when the MAIL FROM
              domain is listed with any A record under rbl_domain.
              The maps_rbl_reject_code parameter specifies the response code
              for rejected requests (default:  554); the default_rbl_reply
              parameter specifies the default server reply; and the
              rbl_reply_maps parameter specifies tables with server replies
              indexed by rbl_domain.  This feature is available in Postfix 2.0
              and later.

       reject_sender_login_mismatch
              Reject the request when $smtpd_sender_login_maps specifies an
              owner for the MAIL FROM address, but the client is not (SASL)
              logged in as that MAIL FROM address owner; or when the client is
              (SASL) logged in, but the client login name doesn't own the MAIL
              FROM address according to $smtpd_sender_login_maps.

       reject_unauthenticated_sender_login_mismatch
              Enforces the reject_sender_login_mismatch restriction for
              unauthenticated clients only. This feature is available in
              Postfix version 2.1 and later.

       reject_unknown_sender_domain
              Reject the request when Postfix is not the final destination for
              the sender address, and the MAIL FROM domain has 1) no DNS MX
              and no DNS A record, or 2) a malformed MX record such as a
              record with a zero-length MX hostname (Postfix version 2.3 and
              later).
              The reply is specified with the unknown_address_reject_code
              parameter (default: 450), unknown_address_tempfail_action
              (default: defer_if_permit), or 550 (nullmx, Postfix 3.0 and
              later). See the respective parameter descriptions for details.

       reject_unlisted_sender
              Reject the request when the MAIL FROM address is not listed in
              the list of valid recipients for its domain class. See the
              smtpd_reject_unlisted_sender parameter description for details.
              This feature is available in Postfix 2.1 and later.

       reject_unverified_sender
              Reject the request when mail to the MAIL FROM address is known
              to bounce, or when the sender address destination is not
              reachable.  Address verification information is managed by the
              verify(8) server; see the ADDRESS_VERIFICATION_README file for
              details.
              The unverified_sender_reject_code parameter specifies the
              numerical response code when an address is known to bounce
              (default: 450, change into 550 when you are confident that it is
              safe to do so).
              The unverified_sender_defer_code specifies the numerical
              response code when an address probe failed due to a temporary
              problem (default: 450).
              The unverified_sender_tempfail_action parameter specifies the
              action after address probe failure due to a temporary problem
              (default: defer_if_permit).
              This feature breaks for aliased addresses with
              "enable_original_recipient = no" (Postfix <= 3.2).
              This feature is available in Postfix 2.1 and later.

       Other restrictions that are valid in this context:

       ⊕      Generic restrictions that can be used in any SMTP command
              context, described under smtpd_client_restrictions.

       ⊕      SMTP command specific restrictions described under
              smtpd_client_restrictions and smtpd_helo_restrictions.

       ⊕      SMTP command specific restrictions described under
              smtpd_recipient_restrictions. When recipient restrictions are
              listed under smtpd_sender_restrictions, they have effect only
              with "smtpd_delay_reject = yes", so that
              $smtpd_sender_restrictions is evaluated at the time of the RCPT
              TO command.

       Examples:

       smtpd_sender_restrictions = reject_unknown_sender_domain
       smtpd_sender_restrictions = reject_unknown_sender_domain,
           check_sender_access hash:/etc/postfix/access

smtpd_service_name (default: smtpd)
       The internal service that postscreen(8) hands off allowed connections
       to. In a future version there may be different classes of SMTP service.

       This feature is available in Postfix 2.8.

smtpd_soft_error_limit (default: 10)
       The number of errors a remote SMTP client is allowed to make without
       delivering mail before the Postfix SMTP server slows down all its
       responses.

       ⊕      With Postfix version 2.1 and later, when the error count is >
              $smtpd_soft_error_limit, the Postfix SMTP server delays all
              responses by $smtpd_error_sleep_time.

       ⊕      With Postfix versions 2.0 and earlier, when the error count is >
              $smtpd_soft_error_limit, the Postfix SMTP server delays all
              responses by the larger of (number of errors) seconds or
              $smtpd_error_sleep_time.

       ⊕      With Postfix versions 2.0 and earlier, when the error count is
              <= $smtpd_soft_error_limit, the Postfix SMTP server delays 4XX
              and 5XX responses by $smtpd_error_sleep_time.

smtpd_starttls_timeout (default: see postconf -d output)
       The time limit for Postfix SMTP server write and read operations during
       TLS startup and shutdown handshake procedures. The current default
       value is stress-dependent. Before Postfix version 2.8, it was fixed at
       300s.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.2 and later.

smtpd_timeout (default: normal: 300s, overload: 10s)
       When the Postfix SMTP server wants to send an SMTP server response, how
       long the Postfix SMTP server will wait for an underlying network write
       operation to complete; and when the Postfix SMTP server Postfix wants
       to receive an SMTP client request, how long the Postfix SMTP server
       will wait for an underlying network read operation to complete. See the
       smtpd_per_request_deadline for how this time limit may be enforced
       (with Postfix 2.9-3.6 see smtpd_per_record_deadline).

       Normally the default limit is 300s, but it changes under overload to
       just 10s. With Postfix 2.5 and earlier, the SMTP server always uses a
       time limit of 300s by default.

       Note: if you set SMTP time limits to very large values you may have to
       update the global ipc_timeout parameter.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

smtpd_tls_CAfile (default: empty)
       A file containing (PEM format) CA certificates of root CAs trusted to
       sign either remote SMTP client certificates or intermediate CA
       certificates.  These are loaded into memory before the smtpd(8) server
       enters the chroot jail. If the number of trusted roots is large,
       consider using smtpd_tls_CApath instead, but note that the latter
       directory must be present in the chroot jail if the smtpd(8) server is
       chrooted. This file may also be used to augment the server certificate
       trust chain, but it is best to include all the required certificates
       directly in the server certificate file.

       Specify "smtpd_tls_CAfile = /path/to/system_CA_file" to use ONLY the
       system-supplied default Certification Authority certificates.

       Specify "tls_append_default_CA = no" to prevent Postfix from appending
       the system-supplied default CAs and trusting third-party certificates.

       By default (see smtpd_tls_ask_ccert), client certificates are not
       requested, and smtpd_tls_CAfile should remain empty. If you do make use
       of client certificates, the distinguished names (DNs) of the
       Certification Authorities listed in smtpd_tls_CAfile are sent to the
       remote SMTP client in the client certificate request message. MUAs with
       multiple client certificates may use the list of preferred
       Certification Authorities to select the correct client certificate.
       You may want to put your "preferred" CA or CAs in this file, and
       install other trusted CAs in $smtpd_tls_CApath.

       Example:

       smtpd_tls_CAfile = /etc/postfix/CAcert.pem

       This feature is available in Postfix 2.2 and later.

smtpd_tls_CApath (default: empty)
       A directory containing (PEM format) CA certificates of root CAs trusted
       to sign either remote SMTP client certificates or intermediate CA
       certificates. Do not forget to create the necessary "hash" links with,
       for example, "$OPENSSL_HOME/bin/c_rehash /etc/postfix/certs". To use
       smtpd_tls_CApath in chroot mode, this directory (or a copy) must be
       inside the chroot jail.

       Specify "smtpd_tls_CApath = /path/to/system_CA_directory" to use ONLY
       the system-supplied default Certification Authority certificates.

       Specify "tls_append_default_CA = no" to prevent Postfix from appending
       the system-supplied default CAs and trusting third-party certificates.

       By default (see smtpd_tls_ask_ccert), client certificates are not
       requested, and smtpd_tls_CApath should remain empty. In contrast to
       smtpd_tls_CAfile, DNs of Certification Authorities installed in
       $smtpd_tls_CApath are not included in the client certificate request
       message. MUAs with multiple client certificates may use the list of
       preferred Certification Authorities to select the correct client
       certificate.  You may want to put your "preferred" CA or CAs in
       $smtpd_tls_CAfile, and install the remaining trusted CAs in
       $smtpd_tls_CApath.

       Example:

       smtpd_tls_CApath = /etc/postfix/certs

       This feature is available in Postfix 2.2 and later.

smtpd_tls_always_issue_session_ids (default: yes)
       Force the Postfix SMTP server to issue a TLS session id, even when TLS
       session caching is turned off (smtpd_tls_session_cache_database is
       empty). This behavior is compatible with Postfix < 2.3.

       With Postfix 2.3 and later the Postfix SMTP server can disable session
       id generation when TLS session caching is turned off. This keeps remote
       SMTP clients from caching sessions that almost certainly cannot be
       re-used.

       By default, the Postfix SMTP server always generates TLS session ids.
       This works around a known defect in mail client applications such as MS
       Outlook, and may also prevent interoperability issues with other MTAs.

       Example:

       smtpd_tls_always_issue_session_ids = no

       This feature is available in Postfix 2.3 and later.

smtpd_tls_ask_ccert (default: no)
       Ask a remote SMTP client for a client certificate. This information is
       needed for certificate based mail relaying with, for example, the
       permit_tls_clientcerts feature.

       Some clients such as Netscape will either complain if no certificate is
       available (for the list of CAs in $smtpd_tls_CAfile) or will offer
       multiple client certificates to choose from. This may be annoying, so
       this option is "off" by default.

       This feature is available in Postfix 2.2 and later.

smtpd_tls_auth_only (default: no)
       When TLS encryption is optional in the Postfix SMTP server, do not
       announce or accept SASL authentication over unencrypted connections.

       This feature is available in Postfix 2.2 and later.

smtpd_tls_ccert_verifydepth (default: 9)
       The verification depth for remote SMTP client certificates. A depth of
       1 is sufficient if the issuing CA is listed in a local CA file.

       The default verification depth is 9 (the OpenSSL default) for
       compatibility with earlier Postfix behavior. Prior to Postfix 2.5, the
       default value was 5, but the limit was not actually enforced. If you
       have set this to a lower non-default value, certificates with longer
       trust chains may now fail to verify. Certificate chains with 1 or 2 CAs
       are common, deeper chains are more rare and any number between 5 and 9
       should suffice in practice. You can choose a lower number if, for
       example, you trust certificates directly signed by an issuing CA but
       not any CAs it delegates to.

       This feature is available in Postfix 2.2 and later.

smtpd_tls_cert_file (default: empty)
       File with the Postfix SMTP server RSA certificate in PEM format.  This
       file may also contain the Postfix SMTP server private RSA key.  With
       Postfix >= 3.4 the preferred way to configure server keys and
       certificates is via the "smtpd_tls_chain_files" parameter.

       Public Internet MX hosts without certificates signed by a "reputable"
       CA must generate, and be prepared to present to most clients, a
       self-signed or private-CA signed certificate. The client will not be
       able to authenticate the server, but unless it is running Postfix 2.3
       or similar software, it will still insist on a server certificate.

       For servers that are not public Internet MX hosts, Postfix supports
       configurations with no certificates. This entails the use of just the
       anonymous TLS ciphers, which are not supported by typical SMTP clients.
       Since some clients may not fall back to plain text after a TLS
       handshake failure, a certificate-less Postfix SMTP server will be
       unable to receive email from some TLS-enabled clients. To avoid
       accidental configurations with no certificates, Postfix enables
       certificate-less operation only when the administrator explicitly sets
       "smtpd_tls_cert_file = none". This ensures that new Postfix SMTP server
       configurations will not accidentally enable TLS without certificates.

       Note that server certificates are not optional in TLS 1.3. To run
       without certificates you'd have to disable the TLS 1.3 protocol by
       including '!TLSv1.3' in "smtpd_tls_protocols" and perhaps also
       "smtpd_tls_mandatory_protocols".  It is simpler instead to just
       configure a certificate chain.  Certificate-less operation is not
       recommended.

       Both RSA and DSA certificates are supported.  When both types are
       present, the cipher used determines which certificate will be presented
       to the client.  For Netscape and OpenSSL clients without special cipher
       choices the RSA certificate is preferred.

       To enable a remote SMTP client to verify the Postfix SMTP server
       certificate, the issuing CA certificates must be made available to the
       client. You should include the required certificates in the server
       certificate file, the server certificate first, then the issuing CA(s)
       (bottom-up order).

       Example: the certificate for "server.example.com" was issued by
       "intermediate CA" which itself has a certificate of "root CA".  Create
       the server.pem file with "cat server_cert.pem intermediate_CA.pem
       root_CA.pem > server.pem".

       If you also want to verify client certificates issued by these CAs, you
       can add the CA certificates to the smtpd_tls_CAfile, in which case it
       is not necessary to have them in the smtpd_tls_cert_file,
       smtpd_tls_dcert_file (obsolete) or smtpd_tls_eccert_file.

       A certificate supplied here must be usable as an SSL server certificate
       and hence pass the "openssl verify -purpose sslserver ..." test.

       Example:

       smtpd_tls_cert_file = /etc/postfix/server.pem

       This feature is available in Postfix 2.2 and later.

smtpd_tls_chain_files (default: empty)
       List of one or more PEM files, each holding one or more private keys
       directly followed by a corresponding certificate chain.  The file names
       are separated by commas and/or whitespace.  This parameter obsoletes
       the legacy algorithm-specific key and certificate file settings.  When
       this parameter is non-empty, the legacy parameters are ignored, and a
       warning is logged if any are also non-empty.

       With the proliferation of multiple private key algorithms-which, as of
       OpenSSL 1.1.1, include DSA (obsolete), RSA, ECDSA, Ed25519 and Ed448-it
       is increasingly impractical to use separate parameters to configure the
       key and certificate chain for each algorithm.  Therefore, Postfix now
       supports storing multiple keys and corresponding certificate chains in
       a single file or in a set of files.

       Each key must appear immediately before the corresponding certificate,
       optionally followed by additional issuer certificates that complete the
       certificate chain for that key.  When multiple files are specified,
       they are equivalent to a single file that is concatenated from those
       files in the given order.  Thus, while a key must always precede its
       certificate and issuer chain, it can be in a separate file, so long as
       that file is listed immediately before the file that holds the
       corresponding certificate chain.  Once all the files are concatenated,
       the sequence of PEM objects must be: key1, cert1, [chain1], key2,
       cert2, [chain2], ..., keyN, certN, [chainN].

       Storing the private key in the same file as the corresponding
       certificate is more reliable.  With the key and certificate in separate
       files, there is a chance that during key rollover a Postfix process
       might load a private key and certificate from separate files that don't
       match.  Various operational errors may even result in a persistent
       broken configuration in which the certificate does not match the
       private key.

       The file or files must contain at most one key of each type.  If, for
       example, two or more RSA keys and corresponding chains are listed,
       depending on the version of OpenSSL either only the last one will be
       used or a configuration error may be detected.  Note that while
       "Ed25519" and "Ed448" are considered separate algorithms, the various
       ECDSA curves (typically one of prime256v1, secp384r1 or secp521r1) are
       considered as different parameters of a single "ECDSA" algorithm, so it
       is not presently possible to configure keys for more than one ECDSA
       curve.

       RSA is still the most widely supported algorithm.  Presently (late
       2018), ECDSA support is common, but not yet universal, and Ed25519 and
       Ed448 support is mostly absent.  Therefore, an RSA key should generally
       be configured, along with any additional keys for the other algorithms
       when desired.

       Example (separate files for each key and corresponding certificate
       chain):

           /etc/postfix/main.cf:
               smtpd_tls_chain_files =
                   ${config_directory}/ed25519.pem,
                   ${config_directory}/ed448.pem,
                   ${config_directory}/rsa.pem

           /etc/postfix/ed25519.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----

           /etc/postfix/ed448.pem:
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----

           /etc/postfix/rsa.pem:
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       Example (all keys and certificates in a single file):

           /etc/postfix/main.cf:
               smtpd_tls_chain_files = ${config_directory}/chains.pem

           /etc/postfix/chains.pem:
               -----BEGIN PRIVATE KEY-----
               MC4CAQAwBQYDK2VwBCIEIEJfbbO4BgBQGBg9NAbIJaDBqZb4bC4cOkjtAH+Efbz3
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBKzCB3qADAgECAhQaw+rflRreYuUZBp0HuNn/e5rMZDAFBgMrZXAwFDESMBAG
               ...
               nC0egv51YPDWxEHom4QA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MEcCAQAwBQYDK2VxBDsEOQf+m0P+G0qi+NZ0RolyeiE5zdlPQR8h8y4jByBifpIe
               LNler7nzHQJ1SLcOiXFHXlxp/84VZuh32A==
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIBdjCB96ADAgECAhQSv4oP972KypOZPNPF4fmsiQoRHzAFBgMrZXEwFDESMBAG
               ...
               pQcWsx+4J29e6YWH3Cy/CdUaexKP4RPCZDrPX7bk5C2BQ+eeYOxyThMA
               -----END CERTIFICATE-----
               -----BEGIN PRIVATE KEY-----
               MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQDc4QusgkahH9rL
               ...
               ahQkZ3+krcaJvDSMgvu0tDc=
               -----END PRIVATE KEY-----
               -----BEGIN CERTIFICATE-----
               MIIC+DCCAeCgAwIBAgIUIUkrbk1GAemPCT8i9wKsTGDH7HswDQYJKoZIhvcNAQEL
               ...
               Rirz15HGVNTK8wzFd+nulPzwUo6dH2IU8KazmyRi7OGvpyrMlm15TRE2oyE=
               -----END CERTIFICATE-----

       This feature is available in Postfix 3.4 and later.

smtpd_tls_cipherlist (default: empty)
       Obsolete Postfix < 2.3 control for the Postfix SMTP server TLS cipher
       list. It is easy to create interoperability problems by choosing a
       non-default cipher list. Do not use a non-default TLS cipherlist for MX
       hosts on the public Internet. Clients that begin the TLS handshake, but
       are unable to agree on a common cipher, may not be able to send any
       email to the SMTP server. Using a restricted cipher list may be more
       appropriate for a dedicated MSA or an internal mailhub, where one can
       exert some control over the TLS software and settings of the connecting
       clients.

       Note: do not use "" quotes around the parameter value.

       This feature is available with Postfix version 2.2. It is not used with
       Postfix 2.3 and later; use smtpd_tls_mandatory_ciphers instead.

smtpd_tls_ciphers (default: medium)
       The minimum TLS cipher grade that the Postfix SMTP server will use with
       opportunistic TLS encryption. Cipher types listed in
       smtpd_tls_exclude_ciphers are excluded from the base definition of the
       selected cipher grade.  The default value is "medium" for Postfix
       releases after the middle of 2015, "export" for older releases.

       When TLS is mandatory the cipher grade is chosen via the
       smtpd_tls_mandatory_ciphers configuration parameter, see there for
       syntax details.

       This feature is available in Postfix 2.6 and later. With earlier
       Postfix releases only the smtpd_tls_mandatory_ciphers parameter is
       implemented, and opportunistic TLS always uses "export" or better (i.e.
       all) ciphers.

smtpd_tls_dcert_file (default: empty)
       File with the Postfix SMTP server DSA certificate in PEM format.  This
       file may also contain the Postfix SMTP server private DSA key.  The DSA
       algorithm is obsolete and should not be used.

       See the discussion under smtpd_tls_cert_file for more details.

       Example:

       smtpd_tls_dcert_file = /etc/postfix/server-dsa.pem

       This feature is available in Postfix 2.2 and later.

smtpd_tls_dh1024_param_file (default: empty)
       File with DH parameters that the Postfix SMTP server should use with
       non-export EDH ciphers.

       With Postfix >= 3.7, built with OpenSSL version is 3.0.0 or later, if
       the parameter value is either empty or "auto", then the DH parameter
       selection is delegated to the OpenSSL library, which selects
       appropriate parameters based on the TLS handshake.  This choice is
       likely to be the most interoperable with SMTP clients using various TLS
       libraries, and custom local parameters are no longer recommended when
       using Postfix >= 3.7 built against OpenSSL 3.0.0.

       The best-practice choice of parameters uses a 2048-bit prime.  This is
       fine, despite the historical "1024" in the parameter name.  Do not be
       tempted to use much larger values, performance degrades quickly, and
       you may also cease to interoperate with some mainstream SMTP clients.
       As of Postfix 3.1, the compiled-in default prime is 2048-bits, and it
       is not strictly necessary, though perhaps somewhat beneficial to
       generate custom DH parameters.

       Instead of using the exact same parameter sets as distributed with
       other TLS packages, it is more secure to generate your own set of
       parameters with something like the following commands:

           openssl dhparam -out /etc/postfix/dh2048.pem 2048
           openssl dhparam -out /etc/postfix/dh1024.pem 1024
           # As of Postfix 3.6, export-grade 512-bit DH parameters are no longer
           # supported or needed.
           openssl dhparam -out /etc/postfix/dh512.pem 512

       It is safe to share the same DH parameters between multiple Postfix
       instances.  If you prefer, you can generate separate parameters for
       each instance.

       If you want to take maximal advantage of ciphers that offer forward
       secrecy see the Getting started section of FORWARD_SECRECY_README.  The
       full document conveniently presents all information about Postfix
       "perfect" forward secrecy support in one place: what forward secrecy
       is, how to tweak settings, and what you can expect to see when Postfix
       uses ciphers with forward secrecy.

       Example:

       smtpd_tls_dh1024_param_file = /etc/postfix/dh2048.pem

       This feature is available in Postfix 2.2 and later.

smtpd_tls_dh512_param_file (default: empty)
       File with DH parameters that the Postfix SMTP server should use with
       export-grade EDH ciphers.  The default SMTP server cipher grade is
       "medium" with Postfix releases after the middle of 2015, and as a
       result export-grade cipher suites are by default not used.

       With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no
       longer supported, and this parameter is silently ignored.

       See also the discussion under the smtpd_tls_dh1024_param_file
       configuration parameter.

       Example:

       smtpd_tls_dh512_param_file = /etc/postfix/dh_512.pem

       This feature is available in Postfix 2.2 and later, but is ignored in
       Postfix 3.6 and later.

smtpd_tls_dkey_file (default: $smtpd_tls_dcert_file)
       File with the Postfix SMTP server DSA private key in PEM format.  This
       file may be combined with the Postfix SMTP server DSA certificate file
       specified with $smtpd_tls_dcert_file. The DSA algorithm is obsolete and
       should not be used.

       The private key must be accessible without a pass-phrase, i.e. it must
       not be encrypted. File permissions should grant read-only access to the
       system superuser account ("root"), and no access to anyone else.

       This feature is available in Postfix 2.2 and later.

smtpd_tls_eccert_file (default: empty)
       File with the Postfix SMTP server ECDSA certificate in PEM format.
       This file may also contain the Postfix SMTP server private ECDSA key.
       With Postfix >= 3.4 the preferred way to configure server keys and
       certificates is via the "smtpd_tls_chain_files" parameter.

       See the discussion under smtpd_tls_cert_file for more details.

       Example:

       smtpd_tls_eccert_file = /etc/postfix/ecdsa-scert.pem

       This feature is available in Postfix 2.6 and later, when Postfix is
       compiled and linked with OpenSSL 1.0.0 or later.

smtpd_tls_eckey_file (default: $smtpd_tls_eccert_file)
       File with the Postfix SMTP server ECDSA private key in PEM format.
       This file may be combined with the Postfix SMTP server ECDSA
       certificate file specified with $smtpd_tls_eccert_file.  With Postfix
       >= 3.4 the preferred way to configure server keys and certificates is
       via the "smtpd_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must
       not be encrypted. File permissions should grant read-only access to the
       system superuser account ("root"), and no access to anyone else.

       This feature is available in Postfix 2.6 and later, when Postfix is
       compiled and linked with OpenSSL 1.0.0 or later.

smtpd_tls_eecdh_grade (default: see postconf -d output)
       The Postfix SMTP server security grade for ephemeral elliptic-curve
       Diffie-Hellman (EECDH) key exchange.   As of Postfix 3.6, the value of
       this parameter is always ignored, and Postfix behaves as though the
       auto value (described below) was chosen.

       The available choices are:

       auto   Use the most preferred curve that is supported by both the
              client and the server.  This setting requires Postfix >= 3.2
              compiled and linked with OpenSSL >= 1.0.2.  This is the default
              setting under the above conditions (and the only setting used
              with Postfix >= 3.6).

       none   Don't use EECDH. Ciphers based on EECDH key exchange will be
              disabled. This is the default in Postfix versions 2.6 and 2.7.

       strong Use EECDH with approximately 128 bits of security at a
              reasonable computational cost. This is the default in Postfix
              versions 2.8-3.5.

       ultra  Use EECDH with approximately 192 bits of security at
              computational cost that is approximately twice as high as 128
              bit strength ECC.

       If you want to take maximal advantage of ciphers that offer forward
       secrecy see the Getting started section of FORWARD_SECRECY_README.  The
       full document conveniently presents all information about Postfix
       "perfect" forward secrecy support in one place: what forward secrecy
       is, how to tweak settings, and what you can expect to see when Postfix
       uses ciphers with forward secrecy.

       This feature is available in Postfix 2.6 and later, when it is compiled
       and linked with OpenSSL 1.0.0 or later on platforms where EC algorithms
       have not been disabled by the vendor.

smtpd_tls_exclude_ciphers (default: empty)
       List of ciphers or cipher types to exclude from the SMTP server cipher
       list at all TLS security levels. Excluding valid ciphers can create
       interoperability problems. DO NOT exclude ciphers unless it is
       essential to do so. This is not an OpenSSL cipherlist; it is a simple
       list separated by whitespace and/or commas. The elements are a single
       cipher, or one or more "+" separated cipher properties, in which case
       only ciphers matching all the properties are excluded.

       Examples (some of these will cause problems):

           smtpd_tls_exclude_ciphers = aNULL
           smtpd_tls_exclude_ciphers = MD5, DES
           smtpd_tls_exclude_ciphers = DES+MD5
           smtpd_tls_exclude_ciphers = AES256-SHA, DES-CBC3-MD5
           smtpd_tls_exclude_ciphers = kEDH+aRSA

       The first setting disables anonymous ciphers. The next setting disables
       ciphers that use the MD5 digest algorithm or the (single) DES
       encryption algorithm. The next setting disables ciphers that use MD5
       and DES together.  The next setting disables the two ciphers
       "AES256-SHA" and "DES-CBC3-MD5". The last setting disables ciphers that
       use "EDH" key exchange with RSA authentication.

       This feature is available in Postfix 2.3 and later.

smtpd_tls_fingerprint_digest (default: see postconf -d output)
       The message digest algorithm to construct remote SMTP
       client-certificate fingerprints or public key fingerprints (Postfix 2.9
       and later) for check_ccert_access and permit_tls_clientcerts.

       The default algorithm is sha256 with Postfix >= 3.6 and the
       compatibility_level set to 3.6 or higher. With Postfix <= 3.5, the
       default algorithm is md5.

       The best-practice algorithm is now sha256. Recent advances in hash
       function cryptanalysis have led to md5 and sha1 being deprecated in
       favor of sha256.  However, as long as there are no known "second
       pre-image" attacks against the older algorithms, their use in this
       context, though not recommended, is still likely safe.

       While additional digest algorithms are often available with OpenSSL's
       libcrypto, only those used by libssl in SSL cipher suites are available
       to Postfix.  You'll likely find support for md5, sha1, sha256 and
       sha512.

       To find the fingerprint of a specific certificate file, with a specific
       digest algorithm, run:

           $ openssl x509 -noout -fingerprint -digest -in certfile.pem

       The text to the right of "=" sign is the desired fingerprint.  For
       example:

           $ openssl x509 -noout -fingerprint -sha256 -in cert.pem
           SHA256 Fingerprint=D4:6A:AB:19:24:...:A6:CB:66:82:C0:8E:9B:EE:29:A8:1A

       To extract the public key fingerprint from an X.509 certificate, you
       need to extract the public key from the certificate and compute the
       appropriate digest of its DER (ASN.1) encoding. With OpenSSL the
       "-pubkey" option of the "x509" command extracts the public key always
       in "PEM" format. We pipe the result to another OpenSSL command that
       converts the key to DER and then to the "dgst" command to compute the
       fingerprint.

       Example:

           $ openssl x509 -in cert.pem -noout -pubkey |
               openssl pkey -pubin -outform DER |
               openssl dgst -sha256 -c
           (stdin)= 64:3f:1f:f6:e5:1e:d4:2a:56:8b:fc:09:1a:61:98:b5:bc:7c:60:58

       The Postfix SMTP server and client log the peer (leaf) certificate
       fingerprint and public key fingerprint when the TLS loglevel is 2 or
       higher.

       Example: client-certificate access table, with sha256 fingerprints:

           /etc/postfix/main.cf:
               smtpd_tls_fingerprint_digest = sha256
               smtpd_client_restrictions =
                   check_ccert_access hash:/etc/postfix/access,
                   reject
           /etc/postfix/access:
               # Action folded to next line...
               AF:88:7C:AD:51:95:6F:36:96:...:01:FB:2E:48:CD:AB:49:25:A2:3B
                   OK
               85:16:78:FD:73:6E:CE:70:E0:...:5F:0D:3C:C8:6D:C4:2C:24:59:E1
                   permit_auth_destination

       This feature is available in Postfix 2.5 and later.

smtpd_tls_key_file (default: $smtpd_tls_cert_file)
       File with the Postfix SMTP server RSA private key in PEM format.  This
       file may be combined with the Postfix SMTP server RSA certificate file
       specified with $smtpd_tls_cert_file.  With Postfix >= 3.4 the preferred
       way to configure server keys and certificates is via the
       "smtpd_tls_chain_files" parameter.

       The private key must be accessible without a pass-phrase, i.e. it must
       not be encrypted. File permissions should grant read-only access to the
       system superuser account ("root"), and no access to anyone else.

smtpd_tls_loglevel (default: 0)
       Enable additional Postfix SMTP server logging of TLS activity.  Each
       logging level also includes the information that is logged at a lower
       logging level.


              0 Disable logging of TLS activity.


              1 Log only a summary message on TLS handshake completion - no
              logging of client certificate trust-chain verification errors if
              client certificate verification is not required.  With Postfix
              2.8 and earlier, log the summary message, peer certificate
              summary information and unconditionally log trust-chain
              verification errors.


              2 Also log levels during TLS negotiation.


              3 Also log hexadecimal and ASCII dump of TLS negotiation
              process.


              4 Also log hexadecimal and ASCII dump of complete transmission
              after STARTTLS.

       Do not use "smtpd_tls_loglevel = 2" or higher except in case of
       problems. Use of loglevel 4 is strongly discouraged.

       This feature is available in Postfix 2.2 and later.

smtpd_tls_mandatory_ciphers (default: medium)
       The minimum TLS cipher grade that the Postfix SMTP server will use with
       mandatory TLS encryption. The default grade ("medium") is sufficiently
       strong that any benefit from globally restricting TLS sessions to a
       more stringent grade is likely negligible, especially given the fact
       that many implementations still do not offer any stronger ("high"
       grade) ciphers, while those that do, will always use "high" grade
       ciphers. So insisting on "high" grade ciphers is generally
       counter-productive. Allowing "export" or "low" ciphers is typically not
       a good idea, as systems limited to just these are limited to obsolete
       browsers. No known SMTP clients fail to support at least one "medium"
       or "high" grade cipher.

       The following cipher grades are supported:

       export Enable "EXPORT" grade or stronger OpenSSL ciphers.  The
              underlying cipherlist is specified via the tls_export_cipherlist
              configuration parameter, which you are strongly encouraged not
              to change.  This choice is insecure and SHOULD NOT be used.

       low    Enable "LOW" grade or stronger OpenSSL ciphers. The underlying
              cipherlist is specified via the tls_low_cipherlist configuration
              parameter, which you are strongly encouraged not to change.
              This choice is insecure and SHOULD NOT be used.

       medium Enable "MEDIUM" grade or stronger OpenSSL ciphers. These use
              128-bit or longer symmetric bulk-encryption keys. This is the
              default minimum strength for mandatory TLS encryption. The
              underlying cipherlist is specified via the tls_medium_cipherlist
              configuration parameter, which you are strongly encouraged not
              to change.

       high   Enable only "HIGH" grade OpenSSL ciphers. The underlying
              cipherlist is specified via the tls_high_cipherlist
              configuration parameter, which you are strongly encouraged to
              not change.

       null   Enable only the "NULL" OpenSSL ciphers, these provide
              authentication without encryption.  This setting is only
              appropriate in the rare case that all clients are prepared to
              use NULL ciphers (not normally enabled in TLS clients). The
              underlying cipherlist is specified via the tls_null_cipherlist
              configuration parameter, which you are strongly encouraged not
              to change.

       Cipher types listed in smtpd_tls_mandatory_exclude_ciphers or
       smtpd_tls_exclude_ciphers are excluded from the base definition of the
       selected cipher grade. See smtpd_tls_ciphers for cipher controls that
       apply to opportunistic TLS.

       The underlying cipherlists for grades other than "null" include
       anonymous ciphers, but these are automatically filtered out if the
       server is configured to ask for remote SMTP client certificates.  You
       are very unlikely to need to take any steps to exclude anonymous
       ciphers, they are excluded automatically as required.  If you must
       exclude anonymous ciphers even when Postfix does not need or use peer
       certificates, set "smtpd_tls_exclude_ciphers = aNULL". To exclude
       anonymous ciphers only when TLS is enforced, set
       "smtpd_tls_mandatory_exclude_ciphers = aNULL".

       This feature is available in Postfix 2.3 and later.

smtpd_tls_mandatory_exclude_ciphers (default: empty)
       Additional list of ciphers or cipher types to exclude from the Postfix
       SMTP server cipher list at mandatory TLS security levels.  This list
       works in addition to the exclusions listed with
       smtpd_tls_exclude_ciphers (see there for syntax details).

       This feature is available in Postfix 2.3 and later.

smtpd_tls_mandatory_protocols (default: see postconf -d output)
       TLS protocols accepted by the Postfix SMTP server with mandatory TLS
       encryption.  If the list is empty, the server supports all available
       TLS protocol versions.  A non-empty value is a list of protocol names
       to include or exclude, separated by whitespace, commas or colons.

       The valid protocol names (see SSL_get_version(3)) are "SSLv2", "SSLv3",
       "TLSv1", "TLSv1.1", "TLSv1.2" and "TLSv1.3".  Starting with Postfix
       3.6, the default value is ">=TLSv1", which sets TLS 1.0 as the lowest
       supported TLS protocol version (see below).  Older releases use the "!"
       exclusion syntax, also described below.

       As of Postfix 3.6, the preferred way to limit the range of acceptable
       protocols is to set the lowest acceptable TLS protocol version and/or
       the highest acceptable TLS protocol version.  To set the lower bound
       include an element of the form: ">=version" where version is a either
       one of the TLS protocol names listed above, or a hexadecimal number
       corresponding to the desired TLS protocol version (0301 for TLS 1.0,
       0302 for TLS 1.1, etc.).  For the upper bound, use "<=version".  There
       must be no whitespace between the ">=" or "<=" symbols and the protocol
       name or number.

       Hexadecimal protocol numbers make it possible to specify protocol
       bounds for TLS versions that are known to OpenSSL, but might not be
       known to Postfix.  They cannot be used with the legacy exclusion
       syntax.  Leading "0" or "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent to
       "TLSv1".  Hexadecimal versions unknown to OpenSSL will fail to set the
       upper or lower bound, and a warning will be logged.  Hexadecimal
       versions should only be used when Postfix is linked with some future
       version of OpenSSL that supports TLS 1.4 or later, but Postfix does not
       yet support a symbolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.2 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtpd_tls_mandatory_protocols = >=TLSv1.2, <=0305
           # Allow only TLS 1.2 and up:
           smtpd_tls_mandatory_protocols = >=0x0303

       With Postfix < 3.6 there is no support for a minimum or maximum
       version, and the protocol range is configured via protocol exclusions.
       To require at least TLS 1.0, set "smtpd_tls_mandatory_protocols =
       !SSLv2, !SSLv3".  Listing the protocols to include, rather than
       protocols to exclude, is supported, but not recommended.  The exclusion
       form more accurately matches the underlying OpenSSL interface.

       Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.  Disabling this
       protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch
       releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

       Example:

       # Preferred syntax with Postfix >= 3.6:
       smtpd_tls_mandatory_protocols = >=TLSv1.2, <=TLSv1.3
       # Legacy syntax:
       smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1

       This feature is available in Postfix 2.3 and later.

smtpd_tls_protocols (default: see postconf -d output)
       TLS protocols accepted by the Postfix SMTP server with opportunistic
       TLS encryption. If the list is empty, the server supports all available
       TLS protocol versions.  A non-empty value is a list of protocol names
       to include or exclude, separated by whitespace, commas or colons.

       The valid protocol names (see SSL_get_version(3)) are "SSLv2", "SSLv3",
       "TLSv1", "TLSv1.1", "TLSv1.2" and "TLSv1.3".  Starting with Postfix
       3.6, the default value is ">=TLSv1", which sets TLS 1.0 as the lowest
       supported TLS protocol version (see below).  Older releases use the "!"
       exclusion syntax, also described below.

       As of Postfix 3.6, the preferred way to limit the range of acceptable
       protocols is to set the lowest acceptable TLS protocol version and/or
       the highest acceptable TLS protocol version.  To set the lower bound
       include an element of the form: ">=version" where version is a either
       one of the TLS protocol names listed above, or a hexadecimal number
       corresponding to the desired TLS protocol version (0301 for TLS 1.0,
       0302 for TLS 1.1, etc.).  For the upper bound, use "<=version".  There
       must be no whitespace between the ">=" or "<=" symbols and the protocol
       name or number.

       Hexadecimal protocol numbers make it possible to specify protocol
       bounds for TLS versions that are known to OpenSSL, but might not be
       known to Postfix.  They cannot be used with the legacy exclusion
       syntax.  Leading "0" or "0x" prefixes are supported, but not required.
       Therefore, "301", "0301", "0x301" and "0x0301" are all equivalent to
       "TLSv1".  Hexadecimal versions unknown to OpenSSL will fail to set the
       upper or lower bound, and a warning will be logged.  Hexadecimal
       versions should only be used when Postfix is linked with some future
       version of OpenSSL that supports TLS 1.4 or later, but Postfix does not
       yet support a symbolic name for that protocol version.

       Hexadecimal example (Postfix >= 3.6):

           # Allow only TLS 1.0 through (hypothetical) TLS 1.4, once supported
           # in some future version of OpenSSL (presently a warning is logged).
           smtpd_tls_protocols = >=TLSv1, <=0305
           # Allow only TLS 1.0 and up:
           smtpd_tls_protocols = >=0x0301

       With Postfix < 3.6 there is no support for a minimum or maximum
       version, and the protocol range is configured via protocol exclusions.
       To require at least TLS 1.0, set "smtpd_tls_protocols = !SSLv2,
       !SSLv3".  Listing the protocols to include, rather than protocols to
       exclude, is supported, but not recommended.  The exclusion form more
       accurately matches the underlying OpenSSL interface.

       Support for "TLSv1.3" was introduced in OpenSSL 1.1.1.  Disabling this
       protocol via "!TLSv1.3" is supported since Postfix 3.4 (or patch
       releases >= 3.0.14, 3.1.10, 3.2.7 and 3.3.2).

       Example:
       # Preferred syntax with Postfix >= 3.6:
       smtpd_tls_protocols = >=TLSv1, <=TLSv1.3
       # Legacy syntax:
       smtpd_tls_protocols = !SSLv2, !SSLv3

       This feature is available in Postfix 2.6 and later.

smtpd_tls_received_header (default: no)
       Request that the Postfix SMTP server produces Received:  message
       headers that include information about the protocol and cipher used, as
       well as the remote SMTP client CommonName and client certificate issuer
       CommonName.  This is disabled by default, as the information may be
       modified in transit through other mail servers.  Only information that
       was recorded by the final destination can be trusted.

       This feature is available in Postfix 2.2 and later.

smtpd_tls_req_ccert (default: no)
       With mandatory TLS encryption, require a trusted remote SMTP client
       certificate in order to allow TLS connections to proceed.  This option
       implies "smtpd_tls_ask_ccert = yes".

       When TLS encryption is optional, this setting is ignored with a warning
       written to the mail log.

       This feature is available in Postfix 2.2 and later.

smtpd_tls_security_level (default: empty)
       The SMTP TLS security level for the Postfix SMTP server; when a
       non-empty value is specified, this overrides the obsolete parameters
       smtpd_use_tls and smtpd_enforce_tls. This parameter is ignored with
       "smtpd_tls_wrappermode = yes".

       Specify one of the following security levels:

       none   TLS will not be used.

       may    Opportunistic TLS: announce STARTTLS support to remote SMTP
              clients, but do not require that clients use TLS encryption.

       encrypt
              Mandatory TLS encryption: announce STARTTLS support to remote
              SMTP clients, and require that clients use TLS encryption.
              According to RFC 2487 this MUST NOT be applied in case of a
              publicly-referenced SMTP server. Instead, this option should be
              used only on dedicated servers.

       Note 1: the "fingerprint", "verify" and "secure" levels are not
       supported here.  The Postfix SMTP server logs a warning and uses
       "encrypt" instead.  To verify remote SMTP client certificates, see
       TLS_README for a discussion of the smtpd_tls_ask_ccert,
       smtpd_tls_req_ccert, and permit_tls_clientcerts features.

       Note 2: The parameter setting "smtpd_tls_security_level = encrypt"
       implies "smtpd_tls_auth_only = yes".

       Note 3: when invoked via "sendmail -bs", Postfix will never offer
       STARTTLS due to insufficient privileges to access the server private
       key. This is intended behavior.

       This feature is available in Postfix 2.3 and later.

smtpd_tls_session_cache_database (default: empty)
       Name of the file containing the optional Postfix SMTP server TLS
       session cache. Specify a database type that supports enumeration, such
       as btree or sdbm; there is no need to support concurrent access.  The
       file is created if it does not exist. The smtpd(8) daemon does not use
       this parameter directly, rather the cache is implemented indirectly in
       the tlsmgr(8) daemon. This means that per-smtpd-instance master.cf
       overrides of this parameter are not effective. Note that each of the
       cache databases supported by tlsmgr(8) daemon:
       $smtpd_tls_session_cache_database, $smtp_tls_session_cache_database
       (and with Postfix 2.3 and later $lmtp_tls_session_cache_database),
       needs to be stored separately. It is not at this time possible to store
       multiple caches in a single database.

       Note: dbm databases are not suitable. TLS session objects are too
       large.

       As of version 2.5, Postfix no longer uses root privileges when opening
       this file. The file should now be stored under the Postfix-owned
       data_directory. As a migration aid, an attempt to open the file under a
       non-Postfix directory is redirected to the Postfix-owned
       data_directory, and a warning is logged.

       As of Postfix 2.11 the preferred mechanism for session resumption is
       RFC 5077 TLS session tickets, which don't require server-side storage.
       Consequently, for Postfix >= 2.11 this parameter should generally be
       left empty.  TLS session tickets require an OpenSSL library (at least
       version 0.9.8h) that provides full support for this TLS extension.  See
       also smtpd_tls_session_cache_timeout.

       Example:

       smtpd_tls_session_cache_database = btree:/var/db/postfix/smtpd_scache

       This feature is available in Postfix 2.2 and later.

smtpd_tls_session_cache_timeout (default: 3600s)
       The expiration time of Postfix SMTP server TLS session cache
       information. A cache cleanup is performed periodically every
       $smtpd_tls_session_cache_timeout seconds. As with
       $smtpd_tls_session_cache_database, this parameter is implemented in the
       tlsmgr(8) daemon and therefore per-smtpd-instance master.cf overrides
       are not possible.

       As of Postfix 2.11 this setting cannot exceed 100 days.  If set <= 0,
       session caching is disabled, not just via the database, but also via
       RFC 5077 TLS session tickets, which don't require server-side storage.
       If set to a positive value less than 2 minutes, the minimum value of 2
       minutes is used instead.  TLS session tickets require an OpenSSL
       library (at least version 0.9.8h) that provides full support for this
       TLS extension.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.2 and later, and updated for TLS
       session ticket support in Postfix 2.11.

smtpd_tls_wrappermode (default: no)
       Run the Postfix SMTP server in TLS "wrapper" mode, instead of using the
       STARTTLS command.

       If you want to support this service, enable a special port in
       master.cf, and specify "-o smtpd_tls_wrappermode=yes" on the SMTP
       server's command line. Port 465 (submissions/smtps) is reserved for
       this purpose.

       This feature is available in Postfix 2.2 and later.

smtpd_upstream_proxy_protocol (default: empty)
       The name of the proxy protocol used by an optional before-smtpd proxy
       agent. When a proxy agent is used, this protocol conveys local and
       remote address and port information.  Specify
       "smtpd_upstream_proxy_protocol = haproxy" to enable the haproxy
       protocol; version 2 is supported with Postfix 3.5 and later.

       NOTE: To use the nginx proxy with smtpd(8), enable the XCLIENT protocol
       with smtpd_authorized_xclient_hosts. This supports SASL authentication
       in the proxy agent (Postfix 2.9 and later).

       This feature is available in Postfix 2.10 and later.

smtpd_upstream_proxy_timeout (default: 5s)
       The time limit for the proxy protocol specified with the
       smtpd_upstream_proxy_protocol parameter.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.10 and later.

smtpd_use_tls (default: no)
       Opportunistic TLS: announce STARTTLS support to remote SMTP clients,
       but do not require that clients use TLS encryption.

       Note: when invoked via "sendmail -bs", Postfix will never offer
       STARTTLS due to insufficient privileges to access the server private
       key. This is intended behavior.

       This feature is available in Postfix 2.2 and later. With Postfix 2.3
       and later use smtpd_tls_security_level instead.

smtputf8_autodetect_classes (default: sendmail, verify)
       Detect that a message requires SMTPUTF8 support for the specified mail
       origin classes.  This is a workaround to avoid chicken-and-egg problems
       during the initial SMTPUTF8 roll-out in environments with pre-existing
       mail flows that contain UTF8. Those mail flows should not break because
       Postfix suddenly refuses to deliver such mail to down-stream MTAs that
       don't announce SMTPUTF8 support.

       The problem is that Postfix cannot rely solely on the sender's
       declaration that a message requires SMTPUTF8 support, because UTF8 may
       be introduced during local processing (for example, the client hostname
       in Postfix's Received: header, adding @$myorigin or .$mydomain to an
       incomplete address, address rewriting, alias expansion, automatic BCC
       recipients, local forwarding, and changes made by header checks or
       Milter applications).

       For now, the default is to enable "SMTPUTF8 required" autodetection
       only for Postfix sendmail command-line submissions and address
       verification probes.  This may change once SMTPUTF8 support achieves
       world domination.  However, sites that add UTF8 content via local
       processing (see above) should autodetect the need for SMTPUTF8 support
       for all email.

       Specify one or more of the following:

        sendmail
              Submission with the Postfix sendmail(1) command.

        smtpd Mail received with the smtpd(8) daemon.

        qmqpd Mail received with the qmqpd(8) daemon.

        forward
              Local forwarding or aliasing.  When a message is received with
              "SMTPUTF8 required", then the forwarded (aliased) message always
              has "SMTPUTF8 required".

        bounce
              Submission by the bounce(8) daemon.  When a message is received
              with "SMTPUTF8 required", then the delivery status notification
              always has "SMTPUTF8 required".

        notify
              Postmaster notification from the smtp(8) or smtpd(8) daemon.

        verify
              Address verification probe from the verify(8) daemon.

        all   Enable SMTPUTF8 autodetection for all mail.

       This feature is available in Postfix 3.0 and later.

smtputf8_enable (default: yes)
       Enable preliminary SMTPUTF8 support for the protocols described in RFC
       6531, RFC 6532, and RFC 6533. This requires that Postfix is built to
       support these protocols.

       This feature is available in Postfix 3.0 and later.

soft_bounce (default: no)
       Safety net to keep mail queued that would otherwise be returned to the
       sender.  This parameter disables locally-generated bounces, changes the
       handling of negative responses from remote servers, content filters or
       plugins, and prevents the Postfix SMTP server from rejecting mail
       permanently by changing 5xx reply codes into 4xx.  However, soft_bounce
       is no cure for address rewriting mistakes or mail routing mistakes.

       Note: "soft_bounce = yes" is in some cases implemented by modifying
       server responses. Therefore, the response that Postfix logs may differ
       from the response that Postfix actually sends or receives.

       Example:

       soft_bounce = yes

stale_lock_time (default: 500s)
       The time after which a stale exclusive mailbox lockfile is removed.
       This is used for delivery to file or mailbox.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

stress (default: empty)
       This feature is documented in the STRESS_README document.

       This feature is available in Postfix 2.5 and later.

strict_7bit_headers (default: no)
       Reject mail with 8-bit text in message headers. This blocks mail from
       poorly written applications.

       This feature should not be enabled on a general purpose mail server,
       because it is likely to reject legitimate email.

       This feature is available in Postfix 2.0 and later.

strict_8bitmime (default: no)
       Enable both strict_7bit_headers and strict_8bitmime_body.

       This feature should not be enabled on a general purpose mail server,
       because it is likely to reject legitimate email.

       This feature is available in Postfix 2.0 and later.

strict_8bitmime_body (default: no)
       Reject 8-bit message body text without 8-bit MIME content encoding
       information.  This blocks mail from poorly written applications.

       Unfortunately, this also rejects majordomo approval requests when the
       included request contains valid 8-bit MIME mail, and it rejects bounces
       from mailers that do not MIME encapsulate 8-bit content (for example,
       bounces from qmail or from old versions of Postfix).

       This feature should not be enabled on a general purpose mail server,
       because it is likely to reject legitimate email.

       This feature is available in Postfix 2.0 and later.

strict_mailbox_ownership (default: yes)
       Defer delivery when a mailbox file is not owned by its recipient.  The
       default setting is not backwards compatible.

       This feature is available in Postfix 2.5.3 and later.

strict_mime_encoding_domain (default: no)
       Reject mail with invalid Content-Transfer-Encoding: information for the
       message/* or multipart/* MIME content types.  This blocks mail from
       poorly written software.

       This feature should not be enabled on a general purpose mail server,
       because it will reject mail after a single violation.

       This feature is available in Postfix 2.0 and later.

strict_rfc821_envelopes (default: no)
       Require that addresses received in SMTP MAIL FROM and RCPT TO commands
       are enclosed with <>, and that those addresses do not contain RFC 822
       style comments or phrases.  This stops mail from poorly written
       software.

       By default, the Postfix SMTP server accepts RFC 822 syntax in MAIL FROM
       and RCPT TO addresses.

strict_smtputf8 (default: no)
       Enable stricter enforcement of the SMTPUTF8 protocol. The Postfix SMTP
       server accepts UTF8 sender or recipient addresses only when the client
       requests an SMTPUTF8 mail transaction.

       This feature is available in Postfix 3.0 and later.

sun_mailtool_compatibility (default: no)
       Obsolete SUN mailtool compatibility feature. Instead, use
       "mailbox_delivery_lock = dotlock".

swap_bangpath (default: yes)
       Enable the rewriting of "site!user" into "user@site".  This is
       necessary if your machine is connected to UUCP networks.  It is enabled
       by default.

       Note: with Postfix version 2.2, message header address rewriting
       happens only when one of the following conditions is true:

       ⊕      The message is received with the Postfix sendmail(1) command,

       ⊕      The message is received from a network client that matches
              $local_header_rewrite_clients,

       ⊕      The message is received from the network, and the
              remote_header_rewrite_domain parameter specifies a non-empty
              value.

       To get the behavior before Postfix version 2.2, specify
       "local_header_rewrite_clients = static:all".

       Example:

       swap_bangpath = no

syslog_facility (default: mail)
       The syslog facility of Postfix logging. Specify a facility as defined
       in syslog.conf(5). The default facility is "mail".

       Warning: a non-default syslog_facility setting takes effect only after
       a Postfix process has completed initialization.  Errors during process
       initialization will be logged with the default facility.  Examples are
       errors while parsing the command line arguments, and errors while
       accessing the Postfix main.cf configuration file.

syslog_name (default: see postconf -d output)
       A prefix that is prepended to the process name in syslog records, so
       that, for example, "smtpd" becomes "prefix/smtpd".

       Warning: a non-default syslog_name setting takes effect only after a
       Postfix process has completed initialization. Errors during process
       initialization will be logged with the default name. Examples are
       errors while parsing the command line arguments, and errors while
       accessing the Postfix main.cf configuration file.

tcp_windowsize (default: 0)
       An optional workaround for routers that break TCP window scaling.
       Specify a value > 0 and < 65536 to enable this feature.  With Postfix
       TCP servers (smtpd(8), qmqpd(8)), this feature is implemented by the
       Postfix master(8) daemon.

       To change this parameter without stopping Postfix, you need to first
       terminate all Postfix TCP servers:

           # postconf -e master_service_disable=inet
           # postfix reload

       This immediately terminates all processes that accept network
       connections.  Next, you enable Postfix TCP servers with the updated
       tcp_windowsize setting:

           # postconf -e tcp_windowsize=65535 master_service_disable=
           # postfix reload

       If you skip these steps with a running Postfix system, then the
       tcp_windowsize change will work only for Postfix TCP clients (smtp(8),
       lmtp(8)).

       This feature is available in Postfix 2.6 and later.

tls_append_default_CA (default: no)
       Append the system-supplied default Certification Authority certificates
       to the ones specified with *_tls_CApath or *_tls_CAfile.  The default
       is "no"; this prevents Postfix from trusting third-party certificates
       and giving them relay permission with permit_tls_all_clientcerts.

       This feature is available in Postfix 2.4.15, 2.5.11, 2.6.8, 2.7.2 and
       later versions. Specify "tls_append_default_CA = yes" for backwards
       compatibility, to avoid breaking certificate verification with sites
       that don't use permit_tls_all_clientcerts.

tls_daemon_random_bytes (default: 32)
       The number of pseudo-random bytes that an smtp(8) or smtpd(8) process
       requests from the tlsmgr(8) server in order to seed its internal pseudo
       random number generator (PRNG).  The default of 32 bytes (equivalent to
       256 bits) is sufficient to generate a 128bit (or 168bit) session key.

       This feature is available in Postfix 2.2 and later.

tls_dane_digest_agility (default: on)
       Configure RFC7671 DANE TLSA digest algorithm agility.  Do not change
       this setting from its default value.

       See Section 8 of RFC7671 for correct key rotation procedures.

       This feature is available in Postfix 2.11 through 3.1.  Postfix 3.2 and
       later ignore this configuration parameter and behave as though it were
       set to "on".

tls_dane_digests (default: sha512 sha256)
       DANE TLSA (RFC 6698, RFC 7671, RFC 7672) resource-record "matching
       type" digest algorithms in descending preference order.  All the
       specified algorithms must be supported by the underlying OpenSSL
       library, otherwise the Postfix SMTP client will not support DANE TLSA
       security.

       Specify a list of digest names separated by commas and/or whitespace.
       Each digest name may be followed by an optional "=<number>" suffix.
       For example, "sha512" may instead be specified as "sha512=2" and
       "sha256" may instead be specified as "sha256=1".  The optional number
       must match the <a
       href="https://www.iana.org/assignments/dane-parameters/dane-parameters.xhtml#matching-types"
       >IANA assigned TLSA matching type number the algorithm in question.
       Postfix will check this constraint for the algorithms it knows about.
       Additional matching type algorithms registered with IANA can be added
       with explicit numbers provided they are supported by OpenSSL.

       Invalid list elements are logged with a warning and disable DANE
       support.  TLSA RRs that specify digests not included in the list are
       ignored with a warning.

       Note: It is unwise to omit sha256 from the digest list.  This digest
       algorithm is the only mandatory to implement digest algorithm in RFC
       6698, and many servers are expected to publish TLSA records with just
       sha256 digests.  Unless one of the standard digests is seriously
       compromised and servers have had ample time to update their TLSA
       records you should not omit any standard digests, just arrange them in
       order from strongest to weakest.

       This feature is available in Postfix 2.11 and later.

tls_dane_trust_anchor_digest_enable (default: yes)
       Enable support for RFC 6698 (DANE TLSA) DNS records that contain
       digests of trust-anchors with certificate usage "2".  Do not change
       this setting from its default value.

       This feature is available in Postfix 2.11 through 3.1.  It has been
       withdrawn in Postfix 3.2, as trust-anchor TLSA records are now widely
       used and have proved sufficiently reliable.  Postfix 3.2 and later
       ignore this configuration parameter and behaves as though it were set
       to "yes".

tls_disable_workarounds (default: see postconf -d output)
       List or bit-mask of OpenSSL bug work-arounds to disable.

       The OpenSSL toolkit includes a set of work-arounds for buggy SSL/TLS
       implementations. Applications, such as Postfix, that want to maximize
       interoperability ask the OpenSSL library to enable the full set of
       recommended work-arounds.

       From time to time, it is discovered that a work-around creates a
       security issue, and should no longer be used. If upgrading OpenSSL to a
       fixed version is not an option or an upgrade is not available in a
       timely manner, or in closed environments where no buggy clients or
       servers exist, it may be appropriate to disable some or all of the
       OpenSSL interoperability work-arounds. This parameter specifies which
       bug work-arounds to disable.

       If the value of the parameter is a hexadecimal long integer starting
       with "0x", the bug work-arounds corresponding to the bits specified in
       its value are removed from the SSL_OP_ALL work-around bit-mask (see
       openssl/ssl.h and SSL_CTX_set_options(3)). You can specify more bits
       than are present in SSL_OP_ALL, excess bits are ignored. Specifying
       0xFFFFFFFF disables all bug-workarounds on a 32-bit system. This should
       also be sufficient on 64-bit systems, until OpenSSL abandons support
       for 32-bit systems and starts using the high 32 bits of a 64-bit
       bug-workaround mask.

       Otherwise, the parameter is a white-space or comma separated list of
       specific named bug work-arounds chosen from the list below. It is
       possible that your OpenSSL version includes new bug work-arounds added
       after your Postfix source code was last updated, in that case you can
       only disable one of these via the hexadecimal syntax above.

       CRYPTOPRO_TLSEXT_BUG
              New with GOST support in OpenSSL 1.0.0.

       DONT_INSERT_EMPTY_FRAGMENTS
              See SSL_CTX_set_options(3)

       LEGACY_SERVER_CONNECT
              See SSL_CTX_set_options(3)

       MICROSOFT_BIG_SSLV3_BUFFER
              See SSL_CTX_set_options(3)

       MICROSOFT_SESS_ID_BUG
              See SSL_CTX_set_options(3)

       MSIE_SSLV2_RSA_PADDING
              also aliased as CVE-2005-2969. Postfix 2.8 disables this
              work-around by default with OpenSSL versions that may predate
              the fix. Fixed in OpenSSL 0.9.7h and OpenSSL 0.9.8a.

       NETSCAPE_CHALLENGE_BUG
              See SSL_CTX_set_options(3)

       NETSCAPE_REUSE_CIPHER_CHANGE_BUG
              also aliased as CVE-2010-4180. Postfix 2.8 disables this
              work-around by default with OpenSSL versions that may predate
              the fix. Fixed in OpenSSL 0.9.8q and OpenSSL 1.0.0c.

       SSLEAY_080_CLIENT_DH_BUG
              See SSL_CTX_set_options(3)

       SSLREF2_REUSE_CERT_TYPE_BUG
              See SSL_CTX_set_options(3)

       TLS_BLOCK_PADDING_BUG
              See SSL_CTX_set_options(3)

       TLS_D5_BUG
              See SSL_CTX_set_options(3)

       TLS_ROLLBACK_BUG
              See SSL_CTX_set_options(3).  This is disabled in OpenSSL 0.9.7
              and later. Nobody should still be using 0.9.6!

       TLSEXT_PADDING
              Postfix >= 3.4. See SSL_CTX_set_options(3).

       This feature is available in Postfix 2.8 and later.

tls_eecdh_auto_curves (default: see postconf -d output)
       The prioritized list of elliptic curves supported by the Postfix SMTP
       client and server.  These curves are used by the Postfix SMTP server
       when "smtpd_tls_eecdh_grade = auto".  The selected curves must be
       implemented by OpenSSL and be standardized for use in TLS (RFC 8422).
       It is unwise to list only "bleeding-edge" curves supported by a small
       subset of clients.  The default list is suitable for most users.

       Postfix skips curve names that are unknown to OpenSSL, or that are
       known but not yet implemented.  This makes it possible to "anticipate"
       support for curves that should be used once they become available.  In
       particular, in some OpenSSL versions, the new RFC 8031 curves "X25519"
       and "X448" may be known by name, but ECDH support for either or both
       may be missing.  These curves may appear in the default value of this
       parameter, even though they'll only be usable with later versions of
       OpenSSL.

       This feature is available in Postfix 3.2 and later, when it is compiled
       and linked with OpenSSL 1.0.2 or later on platforms where EC algorithms
       have not been disabled by the vendor.

tls_eecdh_strong_curve (default: prime256v1)
       The elliptic curve used by the Postfix SMTP server for sensibly strong
       ephemeral ECDH key exchange. This curve is used by the Postfix SMTP
       server when "smtpd_tls_eecdh_grade = strong". The phrase "sensibly
       strong" means approximately 128-bit security based on best known
       attacks. The selected curve must be implemented by OpenSSL (as reported
       by ecparam(1) with the "-list_curves" option) and be one of the curves
       listed in Section 5.1.1 of RFC 8422. You should not generally change
       this setting.  Remote SMTP client implementations must support this
       curve for EECDH key exchange to take place.  It is unwise to choose
       only "bleeding-edge" curves supported by only a small subset of
       clients.

       The default "strong" curve is rated in NSA Suite B for information
       classified up to SECRET.

       Note: elliptic curve names are poorly standardized; different standards
       groups are assigning different names to the same underlying curves.
       The curve with the X9.62 name "prime256v1" is also known under the SECG
       name "secp256r1", but OpenSSL does not recognize the latter name.

       If you want to take maximal advantage of ciphers that offer forward
       secrecy see the Getting started section of FORWARD_SECRECY_README.  The
       full document conveniently presents all information about Postfix
       "perfect" forward secrecy support in one place: what forward secrecy
       is, how to tweak settings, and what you can expect to see when Postfix
       uses ciphers with forward secrecy.

       This feature is available in Postfix 2.6 and later, when it is compiled
       and linked with OpenSSL 1.0.0 or later on platforms where EC algorithms
       have not been disabled by the vendor.

tls_eecdh_ultra_curve (default: secp384r1)
       The elliptic curve used by the Postfix SMTP server for maximally strong
       ephemeral ECDH key exchange. This curve is used by the Postfix SMTP
       server when "smtpd_tls_eecdh_grade = ultra". The phrase "maximally
       strong" means approximately 192-bit security based on best known
       attacks.  This additional strength comes at a significant computational
       cost, most users should instead set "smtpd_tls_eecdh_grade = strong".
       The selected curve must be implemented by OpenSSL (as reported by
       ecparam(1) with the "-list_curves" option) and be one of the curves
       listed in Section 5.1.1 of RFC 8422. You should not generally change
       this setting.  Remote SMTP client implementations must support this
       curve for EECDH key exchange to take place.  It is unwise to choose
       only "bleeding-edge" curves supported by only a small subset of
       clients.

       This default "ultra" curve is rated in NSA Suite B for information
       classified up to TOP SECRET.

       If you want to take maximal advantage of ciphers that offer forward
       secrecy see the Getting started section of FORWARD_SECRECY_README.  The
       full document conveniently presents all information about Postfix
       "perfect" forward secrecy support in one place: what forward secrecy
       is, how to tweak settings, and what you can expect to see when Postfix
       uses ciphers with forward secrecy.

       This feature is available in Postfix 2.6 and later, when it is compiled
       and linked with OpenSSL 1.0.0 or later on platforms where EC algorithms
       have not been disabled by the vendor.

tls_export_cipherlist (default: see postconf -d output)
       The OpenSSL cipherlist for "export" or higher grade ciphers. This
       defines the meaning of the "export" setting in smtpd_tls_ciphers,
       smtpd_tls_mandatory_ciphers, smtp_tls_ciphers,
       smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
       lmtp_tls_mandatory_ciphers.  With Postfix releases before the middle of
       2015 this is the default cipherlist for the opportunistic ("may") TLS
       client security level and also the default cipherlist for the SMTP
       server. You are strongly encouraged not to change this setting.

       This feature is available in Postfix 2.3 and later.

tls_fast_shutdown_enable (default: yes)
       A workaround for implementations that hang Postfix while shutting down
       a TLS session, until Postfix times out. With this enabled, Postfix will
       not wait for the remote TLS peer to respond to a TLS 'close'
       notification. This behavior is recommended for TLSv1.0 and later.

tls_high_cipherlist (default: see postconf -d output)
       The OpenSSL cipherlist for "high" grade ciphers. This defines the
       meaning of the "high" setting in smtpd_tls_ciphers,
       smtpd_tls_mandatory_ciphers, smtp_tls_ciphers,
       smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
       lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change
       this setting.

       This feature is available in Postfix 2.3 and later.

tls_legacy_public_key_fingerprints (default: no)
       A temporary migration aid for sites that use certificate public-key
       fingerprints with Postfix 2.9.0..2.9.5, which use an incorrect
       algorithm. This parameter has no effect on the certificate fingerprint
       support that is available since Postfix 2.2.

       Specify "tls_legacy_public_key_fingerprints = yes" temporarily, pending
       a migration from configuration files with incorrect Postfix
       2.9.0..2.9.5 certificate public-key finger prints, to the correct
       fingerprints used by Postfix 2.9.6 and later.  To compute the correct
       certificate public-key fingerprints, see TLS_README.

       This feature is available in Postfix 2.9.6 and later.

tls_low_cipherlist (default: see postconf -d output)
       The OpenSSL cipherlist for "low" or higher grade ciphers. This defines
       the meaning of the "low" setting in smtpd_tls_ciphers,
       smtpd_tls_mandatory_ciphers, smtp_tls_ciphers,
       smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
       lmtp_tls_mandatory_ciphers. You are strongly encouraged not to change
       this setting.

       This feature is available in Postfix 2.3 and later.

tls_medium_cipherlist (default: see postconf -d output)
       The OpenSSL cipherlist for "medium" or higher grade ciphers. This
       defines the meaning of the "medium" setting in smtpd_tls_ciphers,
       smtpd_tls_mandatory_ciphers, smtp_tls_ciphers,
       smtp_tls_mandatory_ciphers, lmtp_tls_ciphers, and
       lmtp_tls_mandatory_ciphers.  This is the default cipherlist for
       mandatory TLS encryption in the TLS client (with anonymous ciphers
       disabled when verifying server certificates).  This is the default
       cipherlist for opportunistic TLS with Postfix releases after the middle
       of 2015.  You are strongly encouraged not to change this setting.

       This feature is available in Postfix 2.3 and later.

tls_null_cipherlist (default: eNULL:!aNULL)
       The OpenSSL cipherlist for "NULL" grade ciphers that provide
       authentication without encryption. This defines the meaning of the
       "null" setting in smtpd_tls_mandatory_ciphers,
       smtp_tls_mandatory_ciphers and lmtp_tls_mandatory_ciphers.  You are
       strongly encouraged not to change this setting.

       This feature is available in Postfix 2.3 and later.

tls_preempt_cipherlist (default: no)
       With SSLv3 and later, use the Postfix SMTP server's cipher preference
       order instead of the remote client's cipher preference order.

       By default, the OpenSSL server selects the client's most preferred
       cipher that the server supports. With SSLv3 and later, the server may
       choose its own most preferred cipher that is supported (offered) by the
       client. Setting "tls_preempt_cipherlist = yes" enables server cipher
       preferences.

       While server cipher selection may in some cases lead to a more secure
       or performant cipher choice, there is some risk of interoperability
       issues. In the past, some SSL clients have listed lower priority
       ciphers that they did not implement correctly. If the server chooses a
       cipher that the client prefers less, it may select a cipher whose
       client implementation is flawed. Most notably Windows 2003 Microsoft
       Exchange servers have flawed implementations of DES-CBC3-SHA, which
       OpenSSL considers stronger than RC4-SHA.  Enabling server cipher-suite
       selection may create interoperability issues with Windows 2003
       Microsoft Exchange clients.

       This feature is available in Postfix 2.8 and later, in combination with
       OpenSSL 0.9.7 and later.

tls_random_bytes (default: 32)
       The number of bytes that tlsmgr(8) reads from $tls_random_source when
       (re)seeding the in-memory pseudo random number generator (PRNG) pool.
       The default of 32 bytes (256 bits) is good enough for 128bit symmetric
       keys.  If using EGD or a device file, a maximum of 255 bytes is read.

       This feature is available in Postfix 2.2 and later.

tls_random_exchange_name (default: see postconf -d output)
       Name of the pseudo random number generator (PRNG) state file that is
       maintained by tlsmgr(8). The file is created when it does not exist,
       and its length is fixed at 1024 bytes.

       As of version 2.5, Postfix no longer uses root privileges when opening
       this file, and the default file location was changed from
       ${config_directory}/prng_exch to ${data_directory}/prng_exch.  As a
       migration aid, an attempt to open the file under a non-Postfix
       directory is redirected to the Postfix-owned data_directory, and a
       warning is logged.

       This feature is available in Postfix 2.2 and later.

tls_random_prng_update_period (default: 3600s)
       The time between attempts by tlsmgr(8) to save the state of the pseudo
       random number generator (PRNG) to the file specified with
       $tls_random_exchange_name.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.2 and later.

tls_random_reseed_period (default: 3600s)
       The maximal time between attempts by tlsmgr(8) to re-seed the in-memory
       pseudo random number generator (PRNG) pool from external sources.  The
       actual time between re-seeding attempts is calculated using the PRNG,
       and is between 0 and the time specified.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.2 and later.

tls_random_source (default: see postconf -d output)
       The external entropy source for the in-memory tlsmgr(8) pseudo random
       number generator (PRNG) pool. Be sure to specify a non-blocking source.
       If this source is not a regular file, the entropy source type must be
       prepended:  egd:/path/to/egd_socket for a source with EGD compatible
       socket interface, or dev:/path/to/device for a device file.

       Note: on OpenBSD systems specify dev:/dev/arandom when dev:/dev/urandom
       gives timeout errors.

       This feature is available in Postfix 2.2 and later.

tls_server_sni_maps (default: empty)
       Optional lookup tables that map names received from remote SMTP clients
       via the TLS Server Name Indication (SNI) extension to the appropriate
       keys and certificate chains.  This parameter is implemented in the
       Postfix TLS library, and applies to both smtpd(8) and the SMTP server
       mode of tlsproxy(8).

       When this parameter is non-empty, the Postfix SMTP server enables SNI
       extension processing, and logs SNI values that are invalid or don't
       match an entry in the specified tables.  When an entry does match, the
       SNI name is logged as part of the connection summary at log levels 1
       and higher.

       The lookup key is either the verbatim SNI domain name or an ancestor
       domain prefixed with a leading dot.  For internationalized domains, the
       lookup key must be in IDNA 2008 A-label form (as required in the TLS
       SNI extension).

       The syntax of the lookup value is the same as with the
       smtp_tls_chain_files parameter (see there for additional details), but
       here scoped to just TLS connections in which the client sends a
       matching SNI domain name.

       Example:

           /etc/postfix/main.cf:
               #
               # The indexed SNI table must be created with "postmap -F"
               #
               indexed = ${default_database_type}:${config_directory}/
               tls_server_sni_maps = ${indexed}sni

           /etc/postfix/sni:
               #
               # The example.com domain has both an RSA and ECDSA certificate
               # chain.  The chain files MUST start with the private key,
               # with the certificate chain next, starting with the leaf
               # (server) certificate, and then the issuer certificates.
               #
               example.com /etc/postfix/sni-chains/rsa2048.example.com.pem,
                           /etc/postfix/sni-chains/ecdsa-p256.example.com.pem
               #
               # The example.net domain has a wildcard certificate, and two
               # additional DNS names.  So its certificate chain is also used
               # with any subdomain, plus the additional names.
               #
               example.net /etc/postfix/sni-chains/example.net.pem
               .example.net /etc/postfix/sni-chains/example.net.pem
               example.info /etc/postfix/sni-chains/example.net.pem
               example.org /etc/postfix/sni-chains/example.net.pem

       Note that the SNI lookup tables should also have entries for the
       domains that correspond to the Postfix SMTP server's default
       certificate(s). This ensures that the remote SMTP client's TLS SNI
       extension gets a positive response when it specifies one of the Postfix
       SMTP server's default domains, and ensures that the Postfix SMTP server
       will not log an SNI name mismatch for such a domain.  The Postfix SMTP
       server's default certificates are then only used when the client sends
       no SNI or when it sends SNI with a domain that the server knows no
       certificate(s) for.

       The mapping from an SNI domain name to a certificate chain is indirect.
       In the input source files for "cdb", "hash", "btree" or other tables
       that are converted to on-disk indexed files via postmap(1), the value
       specified for each key is a list of filenames.  When postmap(1) is used
       with the -F option, the generated table stores for each lookup key the
       base64-encoded contents of the associated files.  When querying tables
       via postmap -Fq, the table value is decoded from base64, yielding the
       original file content, plus a new line.

       With "regexp", "pcre", "inline", "texthash", "static" and similar
       tables that are interpreted at run-time, and don't have a separate
       source format, the table value is again a list files, that are loaded
       into memory when the table is opened.

       With tables whose content is managed outside of Postfix, such as LDAP,
       MySQL, PostgreSQL, socketmap and tcp, the value must be a concatenation
       of the desired PEM keys and certificate chains, that is then further
       encoded to yield a single-line base64 string.  Creation of such tables
       and secure storage (the value includes private key material) are
       outside the responsibility of Postfix.

       With "socketmap" and "tcp" the data will be transmitted in the clear,
       and there is no query access control, so these are generally unsuitable
       for storing SNI chains.  With LDAP and SQL, you should restrict read
       access and use TLS to protect the sensitive data in transit.

       Typically there is only one private key and its chain of certificates
       starting with the "leaf" certificate corresponding to that key, and
       continuing with the appropriate intermediate issuer CA certificates,
       with each certificate ideally followed by its issuer.  Servers that
       have keys and certificates for more than one algorithm (e.g.  both an
       RSA key and an ECDSA key, or even RSA, ECDSA and Ed25519) can use
       multiple chains concatenated together, with the key always listed
       before the corresponding certificates.

       This feature is available in Postfix 3.4 and later.

tls_session_ticket_cipher (default: Postfix >= 3.0: aes-256-cbc, Postfix <
       3.0: aes-128-cbc)
       Algorithm used to encrypt RFC5077 TLS session tickets.  This algorithm
       must use CBC mode, have a 128-bit block size, and must have a key
       length between 128 and 256 bits.  The default is aes-256-cbc.
       Overriding the default to choose a different algorithm is discouraged.

       Setting this parameter empty disables session ticket support in the
       Postfix SMTP server.  Another way to disable session ticket support is
       via the tls_ssl_options parameter.

       This feature is available in Postfix 3.0 and later.

tls_ssl_options (default: empty)
       List or bit-mask of OpenSSL options to enable.

       The OpenSSL toolkit provides a set of options that applications can
       enable to tune the OpenSSL behavior.  Some of these work around bugs in
       other implementations and are on by default.  You can use the
       tls_disable_workarounds parameter to selectively disable some or all of
       the bug work-arounds, making OpenSSL more strict at the cost of
       non-interoperability with SSL clients or servers that exhibit the bugs.

       Other options are off by default, and typically enable or disable
       features rather than bug work-arounds.  These may be turned on (with
       care) via the tls_ssl_options parameter.  The value is a white-space or
       comma separated list of named options chosen from the list below.  The
       names are not case-sensitive, you can use lower-case if you prefer.
       The upper case values below match the corresponding macro name in the
       ssl.h header file with the SSL_OP_ prefix removed.  It is possible that
       your OpenSSL version includes new options added after your Postfix
       source code was last updated, in that case you can only enable one of
       these via the hexadecimal syntax below.

       You should only enable features via the hexadecimal mask when the need
       to control the feature is critical (to deal with a new vulnerability or
       a serious interoperability problem).  Postfix DOES NOT promise
       backwards compatible behavior with respect to the mask bits.  A feature
       enabled via the mask in one release may be enabled by other means in a
       later release, and the mask bit will then be ignored.  Therefore, use
       of the hexadecimal mask is only a temporary measure until a new Postfix
       or OpenSSL release provides a better solution.

       If the value of the parameter is a hexadecimal long integer starting
       with "0x", the options corresponding to the bits specified in its value
       are enabled (see openssl/ssl.h and SSL_CTX_set_options(3)).  You can
       only enable options not already controlled by other Postfix settings.
       For example, you cannot disable protocols or enable server cipher
       preference.  Do not attempt to enable all features by specifying
       0xFFFFFFFF, this is unlikely to be a good idea.  Some bug work-arounds
       are also valid here, allowing them to be re-enabled if/when they're no
       longer enabled by default.  The supported values include:

       ENABLE_MIDDLEBOX_COMPAT
              Postfix >= 3.4. See SSL_CTX_set_options(3).

       LEGACY_SERVER_CONNECT
              See SSL_CTX_set_options(3).

       NO_TICKET
              Enabled by default when needed in fully-patched Postfix >= 2.7.
              Not needed at all for Postfix >= 2.11, unless for some reason
              you do not want to support TLS session resumption.  Best not set
              explicitly.  See SSL_CTX_set_options(3).

       NO_COMPRESSION
              Disable SSL compression even if supported by the OpenSSL
              library.  Compression is CPU-intensive, and compression before
              encryption does not always improve security.

       NO_RENEGOTIATION
              Postfix >= 3.4.  This can reduce opportunities for a potential
              CPU exhaustion attack.  See SSL_CTX_set_options(3).

       NO_SESSION_RESUMPTION_ON_RENEGOTIATION
              Postfix >= 3.4. See SSL_CTX_set_options(3).

       PRIORITIZE_CHACHA
              Postfix >= 3.4. See SSL_CTX_set_options(3).

       This feature is available in Postfix 2.11 and later.

tls_wildcard_matches_multiple_labels (default: yes)
       Match multiple DNS labels with "*" in wildcard certificates.

       Some mail service providers prepend the customer domain name to a base
       domain for which they have a wildcard TLS certificate.  For example,
       the MX records for example.com hosted by example.net may be:

           example.com. IN MX 0 example.com.mx1.example.net.
           example.com. IN MX 0 example.com.mx2.example.net.

       and the TLS certificate may be for "*.example.net". The "*" then
       corresponds with multiple labels in the mail server domain name.  While
       multi-label wildcards are not widely supported, and are not blessed by
       any standard, there is little to be gained by disallowing their use in
       this context.

       Notes:

       ⊕      In a certificate name, the "*" is special only when it is used
              as the first label.

       ⊕      While Postfix (2.11 or later) can match "*" with multiple domain
              name labels, other implementations likely will not.

       ⊕      Earlier Postfix implementations behave as if
              "tls_wildcard_matches_multiple_labels = no".

       This feature is available in Postfix 2.11 and later.

tlsmgr_service_name (default: tlsmgr)
       The name of the tlsmgr(8) service entry in master.cf. This service
       maintains TLS session caches and other information in support of TLS.

       This feature is available in Postfix 2.11 and later.

tlsproxy_client_CAfile (default: $smtp_tls_CAfile)
       A file containing CA certificates of root CAs trusted to sign either
       remote TLS server certificates or intermediate CA certificates.  See
       smtp_tls_CAfile for further details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_CApath (default: $smtp_tls_CApath)
       Directory with PEM format Certification Authority certificates that the
       Postfix tlsproxy(8) client uses to verify a remote TLS server
       certificate. See smtp_tls_CApath for further details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_cert_file (default: $smtp_tls_cert_file)
       File with the Postfix tlsproxy(8) client RSA certificate in PEM format.
       See smtp_tls_cert_file for further details.  The preferred way to
       configure tlsproxy client keys and certificates is via the
       "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_chain_files (default: $smtp_tls_chain_files)
       Files with the Postfix tlsproxy(8) client keys and certificate chains
       in PEM format. See smtp_tls_chain_files for further details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_dcert_file (default: $smtp_tls_dcert_file)
       File with the Postfix tlsproxy(8) client DSA certificate in PEM format.
       See smtp_tls_dcert_file for further details. DSA is obsolete and should
       not be used.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_dkey_file (default: $smtp_tls_dkey_file)
       File with the Postfix tlsproxy(8) client DSA private key in PEM format.
       See smtp_tls_dkey_file for further details. DSA is obsolete and should
       not be used.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_eccert_file (default: $smtp_tls_eccert_file)
       File with the Postfix tlsproxy(8) client ECDSA certificate in PEM
       format. See smtp_tls_eccert_file for further details. The preferred way
       to configure tlsproxy client keys and certificates is via the
       "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_eckey_file (default: $smtp_tls_eckey_file)
       File with the Postfix tlsproxy(8) client ECDSA private key in PEM
       format. See smtp_tls_eckey_file for further details.  The preferred way
       to configure tlsproxy client keys and certificates is via the
       "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_enforce_tls (default: $smtp_enforce_tls)
       Enforcement mode: require that SMTP servers use TLS encryption.  See
       smtp_enforce_tls for further details. Use
       tlsproxy_client_security_level instead.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_fingerprint_digest (default: $smtp_tls_fingerprint_digest)
       The message digest algorithm used to construct remote TLS server
       certificate fingerprints. See smtp_tls_fingerprint_digest for further
       details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_key_file (default: $smtp_tls_key_file)
       File with the Postfix tlsproxy(8) client RSA private key in PEM format.
       See smtp_tls_key_file for further details. The preferred way to
       configure tlsproxy client keys and certificates is via the
       "tlsproxy_client_chain_files" parameter.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_level (default: $smtp_tls_security_level)
       The default TLS security level for the Postfix tlsproxy(8) client. See
       smtp_tls_security_level for further details.

       This feature is available in Postfix 3.4 - 3.6. It was renamed to
       tlsproxy_client_security_level in Postfix 3.7.

tlsproxy_client_loglevel (default: $smtp_tls_loglevel)
       Enable additional Postfix tlsproxy(8) client logging of TLS activity.
       See smtp_tls_loglevel for further details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_loglevel_parameter (default: smtp_tls_loglevel)
       The name of the parameter that provides the tlsproxy_client_loglevel
       value.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_per_site (default: $smtp_tls_per_site)
       Optional lookup tables with the Postfix tlsproxy(8) client TLS usage
       policy by next-hop destination and by remote TLS server hostname.  See
       smtp_tls_per_site for further details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_policy (default: $smtp_tls_policy_maps)
       Optional lookup tables with the Postfix tlsproxy(8) client TLS security
       policy by next-hop destination. See smtp_tls_policy_maps for further
       details.

       This feature is available in Postfix 3.4 - 3.6. It was renamed to
       tlsproxy_client_policy_maps in Postfix 3.7.

tlsproxy_client_policy_maps (default: $smtp_tls_policy_maps)
       Optional lookup tables with the Postfix tlsproxy(8) client TLS security
       policy by next-hop destination. See smtp_tls_policy_maps for further
       details.

       This feature is available in Postfix 3.7 and later. It was previously
       called tlsproxy_client_policy.

tlsproxy_client_scert_verifydepth (default: $smtp_tls_scert_verifydepth)
       The verification depth for remote TLS server certificates.  See
       smtp_tls_scert_verifydepth for further details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_client_security_level (default: $smtp_tls_security_level)
       The default TLS security level for the Postfix tlsproxy(8) client. See
       smtp_tls_security_level for further details.

       This feature is available in Postfix 3.7 and later. It was previously
       called tlsproxy_client_level.

tlsproxy_client_use_tls (default: $smtp_use_tls)
       Opportunistic mode: use TLS when a remote server announces TLS support.
       See smtp_use_tls for further details. Use
       tlsproxy_client_security_level instead.

       This feature is available in Postfix 3.4 and later.

tlsproxy_enforce_tls (default: $smtpd_enforce_tls)
       Mandatory TLS: announce STARTTLS support to remote SMTP clients, and
       require that clients use TLS encryption. See smtpd_enforce_tls for
       further details. Use tlsproxy_tls_security_level instead.

       This feature is available in Postfix 2.8 and later.

tlsproxy_service_name (default: tlsproxy)
       The name of the tlsproxy(8) service entry in master.cf. This service
       performs plaintext <=> TLS ciphertext conversion.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_CAfile (default: $smtpd_tls_CAfile)
       A file containing (PEM format) CA certificates of root CAs trusted to
       sign either remote SMTP client certificates or intermediate CA
       certificates.  See smtpd_tls_CAfile for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_CApath (default: $smtpd_tls_CApath)
       A directory containing (PEM format) CA certificates of root CAs trusted
       to sign either remote SMTP client certificates or intermediate CA
       certificates. See smtpd_tls_CApath for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_always_issue_session_ids (default:
       $smtpd_tls_always_issue_session_ids)
       Force the Postfix tlsproxy(8) server to issue a TLS session id, even
       when TLS session caching is turned off. See
       smtpd_tls_always_issue_session_ids for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_ask_ccert (default: $smtpd_tls_ask_ccert)
       Ask a remote SMTP client for a client certificate. See
       smtpd_tls_ask_ccert for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_ccert_verifydepth (default: $smtpd_tls_ccert_verifydepth)
       The verification depth for remote SMTP client certificates. A depth of
       1 is sufficient if the issuing CA is listed in a local CA file. See
       smtpd_tls_ccert_verifydepth for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_cert_file (default: $smtpd_tls_cert_file)
       File with the Postfix tlsproxy(8) server RSA certificate in PEM format.
       This file may also contain the Postfix tlsproxy(8) server private RSA
       key.  See smtpd_tls_cert_file for further details.  With Postfix >= 3.4
       the preferred way to configure tlsproxy server keys and certificates is
       via the "tlsproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_chain_files (default: $smtpd_tls_chain_files)
       Files with the Postfix tlsproxy(8) server keys and certificate chains
       in PEM format. See smtpd_tls_chain_files for further details.

       This feature is available in Postfix 3.4 and later.

tlsproxy_tls_ciphers (default: $smtpd_tls_ciphers)
       The minimum TLS cipher grade that the Postfix tlsproxy(8) server will
       use with opportunistic TLS encryption. See smtpd_tls_ciphers for
       further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dcert_file (default: $smtpd_tls_dcert_file)
       File with the Postfix tlsproxy(8) server DSA certificate in PEM format.
       This file may also contain the Postfix tlsproxy(8) server private DSA
       key.  DSA is obsolete and should not be used.  See smtpd_tls_dcert_file
       for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dh1024_param_file (default: $smtpd_tls_dh1024_param_file)
       File with DH parameters that the Postfix tlsproxy(8) server should use
       with non-export EDH ciphers. See smtpd_tls_dh1024_param_file for
       further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dh512_param_file (default: $smtpd_tls_dh512_param_file)
       File with DH parameters that the Postfix tlsproxy(8) server should use
       with export-grade EDH ciphers. See smtpd_tls_dh512_param_file for
       further details.  The default SMTP server cipher grade is "medium" with
       Postfix releases after the middle of 2015, and as a result export-grade
       cipher suites are by default not used.

       With Postfix >= 3.6 export-grade Diffie-Hellman key exchange is no
       longer supported, and this parameter is silently ignored.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_dkey_file (default: $smtpd_tls_dkey_file)
       File with the Postfix tlsproxy(8) server DSA private key in PEM format.
       This file may be combined with the Postfix tlsproxy(8) server DSA
       certificate file specified with $smtpd_tls_dcert_file.  DSA is obsolete
       and should not be used.  See smtpd_tls_dkey_file for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_eccert_file (default: $smtpd_tls_eccert_file)
       File with the Postfix tlsproxy(8) server ECDSA certificate in PEM
       format.  This file may also contain the Postfix tlsproxy(8) server
       private ECDSA key.  See smtpd_tls_eccert_file for further details.
       With Postfix >= 3.4 the preferred way to configure tlsproxy server keys
       and certificates is via the "tlsproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_eckey_file (default: $smtpd_tls_eckey_file)
       File with the Postfix tlsproxy(8) server ECDSA private key in PEM
       format.  This file may be combined with the Postfix tlsproxy(8) server
       ECDSA certificate file specified with $smtpd_tls_eccert_file.  See
       smtpd_tls_eckey_file for further details.  With Postfix >= 3.4 the
       preferred way to configure tlsproxy server keys and certificates is via
       the "tlsproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_eecdh_grade (default: $smtpd_tls_eecdh_grade)
       The Postfix tlsproxy(8) server security grade for ephemeral
       elliptic-curve Diffie-Hellman (EECDH) key exchange. See
       smtpd_tls_eecdh_grade for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_exclude_ciphers (default: $smtpd_tls_exclude_ciphers)
       List of ciphers or cipher types to exclude from the tlsproxy(8) server
       cipher list at all TLS security levels. See smtpd_tls_exclude_ciphers
       for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_fingerprint_digest (default: $smtpd_tls_fingerprint_digest)
       The message digest algorithm to construct remote SMTP
       client-certificate fingerprints. See smtpd_tls_fingerprint_digest for
       further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_key_file (default: $smtpd_tls_key_file)
       File with the Postfix tlsproxy(8) server RSA private key in PEM format.
       This file may be combined with the Postfix tlsproxy(8) server RSA
       certificate file specified with $smtpd_tls_cert_file.  See
       smtpd_tls_key_file for further details.  With Postfix >= 3.4 the
       preferred way to configure tlsproxy server keys and certificates is via
       the "tlsproxy_tls_chain_files" parameter.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_loglevel (default: $smtpd_tls_loglevel)
       Enable additional Postfix tlsproxy(8) server logging of TLS activity.
       Each logging level also includes the information that is logged at a
       lower logging level. See smtpd_tls_loglevel for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_mandatory_ciphers (default: $smtpd_tls_mandatory_ciphers)
       The minimum TLS cipher grade that the Postfix tlsproxy(8) server will
       use with mandatory TLS encryption. See smtpd_tls_mandatory_ciphers for
       further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_mandatory_exclude_ciphers (default:
       $smtpd_tls_mandatory_exclude_ciphers)
       Additional list of ciphers or cipher types to exclude from the
       tlsproxy(8) server cipher list at mandatory TLS security levels.  See
       smtpd_tls_mandatory_exclude_ciphers for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_mandatory_protocols (default: $smtpd_tls_mandatory_protocols)
       The SSL/TLS protocols accepted by the Postfix tlsproxy(8) server with
       mandatory TLS encryption. If the list is empty, the server supports all
       available SSL/TLS protocol versions.  See smtpd_tls_mandatory_protocols
       for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_protocols (default: $smtpd_tls_protocols)
       List of TLS protocols that the Postfix tlsproxy(8) server will exclude
       or include with opportunistic TLS encryption. See smtpd_tls_protocols
       for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_req_ccert (default: $smtpd_tls_req_ccert)
       With mandatory TLS encryption, require a trusted remote SMTP client
       certificate in order to allow TLS connections to proceed.  See
       smtpd_tls_req_ccert for further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_security_level (default: $smtpd_tls_security_level)
       The SMTP TLS security level for the Postfix tlsproxy(8) server; when a
       non-empty value is specified, this overrides the obsolete parameters
       smtpd_use_tls and smtpd_enforce_tls. See smtpd_tls_security_level for
       further details.

       This feature is available in Postfix 2.8 and later.

tlsproxy_tls_session_cache_timeout (default: $smtpd_tls_session_cache_timeout)
       Obsolete expiration time of Postfix tlsproxy(8) server TLS session
       cache information. Since the cache is shared with smtpd(8) and managed
       by tlsmgr(8), there is only one expiration time for the SMTP server
       cache shared by all three services, namely
       smtpd_tls_session_cache_timeout.

       This feature is available in Postfix 2.8 and later.

tlsproxy_use_tls (default: $smtpd_use_tls)
       Opportunistic TLS: announce STARTTLS support to remote SMTP clients,
       but do not require that clients use TLS encryption. See smtpd_use_tls
       for further details. Use tlsproxy_tls_security_level instead.

       This feature is available in Postfix 2.8 and later.

tlsproxy_watchdog_timeout (default: 10s)
       How much time a tlsproxy(8) process may take to process local or remote
       I/O before it is terminated by a built-in watchdog timer.  This is a
       safety mechanism that prevents tlsproxy(8) from becoming non-responsive
       due to a bug in Postfix itself or in system software.  To avoid false
       alarms and unnecessary cache corruption this limit cannot be set under
       10s.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       This feature is available in Postfix 2.8 and later

trace_service_name (default: trace)
       The name of the trace service. This service is implemented by the
       bounce(8) daemon and maintains a record of mail deliveries and produces
       a mail delivery report when verbose delivery is requested with
       "sendmail -v".

       This feature is available in Postfix 2.1 and later.

transport_delivery_slot_cost (default: $default_delivery_slot_cost)
       A transport-specific override for the default_delivery_slot_cost
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: transport_delivery_slot_cost parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_delivery_slot_cost").

transport_delivery_slot_discount (default: $default_delivery_slot_discount)
       A transport-specific override for the default_delivery_slot_discount
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: transport_delivery_slot_discount parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_delivery_slot_discount").

transport_delivery_slot_loan (default: $default_delivery_slot_loan)
       A transport-specific override for the default_delivery_slot_loan
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: transport_delivery_slot_loan parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_delivery_slot_loan").

transport_destination_concurrency_failed_cohort_limit (default:
       $default_destination_concurrency_failed_cohort_limit)
       A transport-specific override for the
       default_destination_concurrency_failed_cohort_limit parameter value,
       where transport is the master.cf name of the message delivery
       transport.

       Note: some transport_destination_concurrency_failed_cohort_limit
       parameters will not show up in "postconf" command output before Postfix
       version 2.9.  This limitation applies to many parameters whose name is
       a combination of a master.cf service name and a built-in suffix (in
       this case: "_destination_concurrency_failed_cohort_limit").

       This feature is available in Postfix 2.5 and later.

transport_destination_concurrency_limit (default:
       $default_destination_concurrency_limit)
       A transport-specific override for the
       default_destination_concurrency_limit parameter value, where transport
       is the master.cf name of the message delivery transport.

       Note: some transport_destination_concurrency_limit parameters will not
       show up in "postconf" command output before Postfix version 2.9.  This
       limitation applies to many parameters whose name is a combination of a
       master.cf service name and a built-in suffix (in this case:
       "_destination_concurrency_limit").

transport_destination_concurrency_negative_feedback (default:
       $default_destination_concurrency_negative_feedback)
       A transport-specific override for the
       default_destination_concurrency_negative_feedback parameter value,
       where transport is the master.cf name of the message delivery
       transport.

       Note: some transport_destination_concurrency_negative_feedback
       parameters will not show up in "postconf" command output before Postfix
       version 2.9.  This limitation applies to many parameters whose name is
       a combination of a master.cf service name and a built-in suffix (in
       this case: "_destination_concurrency_negative_feedback").

       This feature is available in Postfix 2.5 and later.

transport_destination_concurrency_positive_feedback (default:
       $default_destination_concurrency_positive_feedback)
       A transport-specific override for the
       default_destination_concurrency_positive_feedback parameter value,
       where transport is the master.cf name of the message delivery
       transport.

       Note: some transport_destination_concurrency_positive_feedback
       parameters will not show up in "postconf" command output before Postfix
       version 2.9.  This limitation applies to many parameters whose name is
       a combination of a master.cf service name and a built-in suffix (in
       this case: "_destination_concurrency_positive_feedback").

       This feature is available in Postfix 2.5 and later.

transport_destination_rate_delay (default: $default_destination_rate_delay)
       A transport-specific override for the default_destination_rate_delay
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: some transport_destination_rate_delay parameters will not show up
       in "postconf" command output before Postfix version 2.9.  This
       limitation applies to many parameters whose name is a combination of a
       master.cf service name and a built-in suffix (in this case:
       "_destination_rate_delay").

       This feature is available in Postfix 2.5 and later.

transport_destination_recipient_limit (default:
       $default_destination_recipient_limit)
       A transport-specific override for the
       default_destination_recipient_limit parameter value, where transport is
       the master.cf name of the message delivery transport.

       Note: some transport_destination_recipient_limit parameters will not
       show up in "postconf" command output before Postfix version 2.9.  This
       limitation applies to many parameters whose name is a combination of a
       master.cf service name and a built-in suffix (in this case:
       "_destination_recipient_limit").

transport_extra_recipient_limit (default: $default_extra_recipient_limit)
       A transport-specific override for the default_extra_recipient_limit
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: transport_extra_recipient_limit parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_extra_recipient_limit").

transport_initial_destination_concurrency (default:
       $initial_destination_concurrency)
       A transport-specific override for the initial_destination_concurrency
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: some transport_initial_destination_concurrency parameters will
       not show up in "postconf" command output before Postfix version 2.9.
       This limitation applies to many parameters whose name is a combination
       of a master.cf service name and a built-in suffix (in this case:
       "_initial_destination_concurrency").

       This feature is available in Postfix 2.5 and later.

transport_maps (default: empty)
       Optional lookup tables with mappings from recipient address to (message
       delivery transport, next-hop destination).  See transport(5) for
       details.

       Specify zero or more "type:table" lookup tables, separated by
       whitespace or comma. Tables will be searched in the specified order
       until a match is found.  If you use this feature with local files, run
       "postmap /etc/postfix/transport" after making a change.

       Pattern matching of domain names is controlled by the presence or
       absence of "transport_maps" in the parent_domain_matches_subdomains
       parameter value.

       For safety reasons, as of Postfix 2.3 this feature does not allow
       $number substitutions in regular expression maps.

       Examples:

       transport_maps = dbm:/etc/postfix/transport
       transport_maps = hash:/etc/postfix/transport

transport_minimum_delivery_slots (default: $default_minimum_delivery_slots)
       A transport-specific override for the default_minimum_delivery_slots
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: transport_minimum_delivery_slots parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_minimum_delivery_slots").

transport_recipient_limit (default: $default_recipient_limit)
       A transport-specific override for the default_recipient_limit parameter
       value, where transport is the master.cf name of the message delivery
       transport.

       Note: some transport_recipient_limit parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case: "_recipient_limit").

transport_recipient_refill_delay (default: $default_recipient_refill_delay)
       A transport-specific override for the default_recipient_refill_delay
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: transport_recipient_refill_delay parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_recipient_refill_delay").

       This feature is available in Postfix 2.4 and later.

transport_recipient_refill_limit (default: $default_recipient_refill_limit)
       A transport-specific override for the default_recipient_refill_limit
       parameter value, where transport is the master.cf name of the message
       delivery transport.

       Note: transport_recipient_refill_limit parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_recipient_refill_limit").

       This feature is available in Postfix 2.4 and later.

transport_retry_time (default: 60s)
       The time between attempts by the Postfix queue manager to contact a
       malfunctioning message delivery transport.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

transport_time_limit (default: $command_time_limit)
       A transport-specific override for the command_time_limit parameter
       value, where transport is the master.cf name of the message delivery
       transport.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       Note: transport_time_limit parameters will not show up in "postconf"
       command output before Postfix version 2.9.  This limitation applies to
       many parameters whose name is a combination of a master.cf service name
       and a built-in suffix (in this case: "_time_limit").

transport_transport_rate_delay (default: $default_transport_rate_delay)
       A transport-specific override for the default_transport_rate_delay
       parameter value, where the initial transport in the parameter name is
       the master.cf name of the message delivery transport.

       Specify a non-negative time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

       Note: transport_transport_rate_delay parameters will not show up in
       "postconf" command output before Postfix version 2.9.  This limitation
       applies to many parameters whose name is a combination of a master.cf
       service name and a built-in suffix (in this case:
       "_transport_rate_delay").

trigger_timeout (default: 10s)
       The time limit for sending a trigger to a Postfix daemon (for example,
       the pickup(8) or qmgr(8) daemon). This time limit prevents programs
       from getting stuck when the mail system is under heavy load.

       Specify a non-zero time value (an integral value plus an optional
       one-letter suffix that specifies the time unit).  Time units: s
       (seconds), m (minutes), h (hours), d (days), w (weeks).  The default
       time unit is s (seconds).

undisclosed_recipients_header (default: see postconf -d output)
       Message header that the Postfix cleanup(8) server inserts when a
       message contains no To: or Cc: message header. With Postfix 2.8 and
       later, the default value is empty. With Postfix 2.4-2.7, specify an
       empty value to disable this feature.

       Example:

       # Default value before Postfix 2.8.
       # Note: the ":" and ";" are both required.
       undisclosed_recipients_header = To: undisclosed-recipients:;

unknown_address_reject_code (default: 450)
       The numerical response code when the Postfix SMTP server rejects a
       sender or recipient address because its domain is unknown.  This is one
       of the possible replies from the restrictions
       reject_unknown_sender_domain and reject_unknown_recipient_domain.

       Do not change this unless you have a complete understanding of RFC
       5321.

unknown_address_tempfail_action (default: $reject_tempfail_action)
       The Postfix SMTP server's action when reject_unknown_sender_domain or
       reject_unknown_recipient_domain fail due to a temporary error
       condition. Specify "defer" to defer the remote SMTP client request
       immediately. With the default "defer_if_permit" action, the Postfix
       SMTP server continues to look for opportunities to reject mail, and
       defers the client request only if it would otherwise be accepted.

       This feature is available in Postfix 2.6 and later.

unknown_client_reject_code (default: 450)
       The numerical Postfix SMTP server response code when a client without
       valid address <=> name mapping is rejected by the
       reject_unknown_client_hostname restriction. The SMTP server always
       replies with 450 when the mapping failed due to a temporary error
       condition.

       Do not change this unless you have a complete understanding of RFC
       5321.

unknown_helo_hostname_tempfail_action (default: $reject_tempfail_action)
       The Postfix SMTP server's action when reject_unknown_helo_hostname
       fails due to a temporary error condition. Specify "defer" to defer the
       remote SMTP client request immediately. With the default
       "defer_if_permit" action, the Postfix SMTP server continues to look for
       opportunities to reject mail, and defers the client request only if it
       would otherwise be accepted.

       This feature is available in Postfix 2.6 and later.

unknown_hostname_reject_code (default: 450)
       The numerical Postfix SMTP server response code when the hostname
       specified with the HELO or EHLO command is rejected by the
       reject_unknown_helo_hostname restriction.

       Do not change this unless you have a complete understanding of RFC
       5321.

unknown_local_recipient_reject_code (default: 550)
       The numerical Postfix SMTP server response code when a recipient
       address is local, and $local_recipient_maps specifies a list of lookup
       tables that does not match the recipient.  A recipient address is local
       when its domain matches $mydestination, $proxy_interfaces or
       $inet_interfaces.

       The default setting is 550 (reject mail) but it is safer to initially
       use 450 (try again later) so you have time to find out if your
       local_recipient_maps settings are OK.

       Example:

       unknown_local_recipient_reject_code = 450

       This feature is available in Postfix 2.0 and later.

unknown_relay_recipient_reject_code (default: 550)
       The numerical Postfix SMTP server reply code when a recipient address
       matches $relay_domains, and relay_recipient_maps specifies a list of
       lookup tables that does not match the recipient address.

       This feature is available in Postfix 2.0 and later.

unknown_virtual_alias_reject_code (default: 550)
       The Postfix SMTP server reply code when a recipient address matches
       $virtual_alias_domains, and $virtual_alias_maps specifies a list of
       lookup tables that does not match the recipient address.

       This feature is available in Postfix 2.0 and later.

unknown_virtual_mailbox_reject_code (default: 550)
       The Postfix SMTP server reply code when a recipient address matches
       $virtual_mailbox_domains, and $virtual_mailbox_maps specifies a list of
       lookup tables that does not match the recipient address.

       This feature is available in Postfix 2.0 and later.

unverified_recipient_defer_code (default: 450)
       The numerical Postfix SMTP server response when a recipient address
       probe fails due to a temporary error condition.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the
       address anyway.

       Do not change this unless you have a complete understanding of RFC
       5321.

       This feature is available in Postfix 2.6 and later.

unverified_recipient_reject_code (default: 450)
       The numerical Postfix SMTP server response when a recipient address is
       rejected by the reject_unverified_recipient restriction.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the
       address anyway.

       Do not change this unless you have a complete understanding of RFC
       5321.

       This feature is available in Postfix 2.1 and later.

unverified_recipient_reject_reason (default: empty)
       The Postfix SMTP server's reply when rejecting mail with
       reject_unverified_recipient. Do not include the numeric SMTP reply code
       or the enhanced status code. By default, the response includes actual
       address verification details.

       Example:

       unverified_recipient_reject_reason = Recipient address lookup failed

       This feature is available in Postfix 2.6 and later.

unverified_recipient_tempfail_action (default: $reject_tempfail_action)
       The Postfix SMTP server's action when reject_unverified_recipient fails
       due to a temporary error condition. Specify "defer" to defer the remote
       SMTP client request immediately. With the default "defer_if_permit"
       action, the Postfix SMTP server continues to look for opportunities to
       reject mail, and defers the client request only if it would otherwise
       be accepted.

       This feature is available in Postfix 2.6 and later.

unverified_sender_defer_code (default: 450)
       The numerical Postfix SMTP server response code when a sender address
       probe fails due to a temporary error condition.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the
       address anyway.

       Do not change this unless you have a complete understanding of RFC
       5321.

       This feature is available in Postfix 2.6 and later.

unverified_sender_reject_code (default: 450)
       The numerical Postfix SMTP server response code when a recipient
       address is rejected by the reject_unverified_sender restriction.

       Unlike elsewhere in Postfix, you can specify 250 in order to accept the
       address anyway.

       Do not change this unless you have a complete understanding of RFC
       5321.

       This feature is available in Postfix 2.1 and later.

unverified_sender_reject_reason (default: empty)
       The Postfix SMTP server's reply when rejecting mail with
       reject_unverified_sender. Do not include the numeric SMTP reply code or
       the enhanced status code. By default, the response includes actual
       address verification details.

       Example:

       unverified_sender_reject_reason = Sender address lookup failed

       This feature is available in Postfix 2.6 and later.

unverified_sender_tempfail_action (default: $reject_tempfail_action)
       The Postfix SMTP server's action when reject_unverified_sender fails
       due to a temporary error condition. Specify "defer" to defer the remote
       SMTP client request immediately. With the default "defer_if_permit"
       action, the Postfix SMTP server continues to look for opportunities to
       reject mail, and defers the client request only if it would otherwise
       be accepted.

       This feature is available in Postfix 2.6 and later.

verp_delimiter_filter (default: -=+)
       The characters Postfix accepts as VERP delimiter characters on the
       Postfix sendmail(1) command line and in SMTP commands.

       This feature is available in Postfix 1.1 and later.

virtual_alias_address_length_limit (default: 1000)
       The maximal length of an email address after virtual alias expansion.
       This stops virtual aliasing loops that increase the address length
       exponentially.

       This feature is available in Postfix 3.0 and later.

virtual_alias_domains (default: $virtual_alias_maps)
       Postfix is the final destination for the specified list of virtual
       alias domains, that is, domains for which all addresses are aliased to
       addresses in other local or remote domains. The SMTP server validates
       recipient addresses with $virtual_alias_maps and rejects non-existent
       recipients. See also the virtual alias domain class in the
       ADDRESS_CLASS_README file

       This feature is available in Postfix 2.0 and later. The default value
       is backwards compatible with Postfix version 1.1.

       The default value is $virtual_alias_maps so that you can keep all
       information about virtual alias domains in one place.  If you have many
       users, it is better to separate information that changes more
       frequently (virtual address -> local or remote address mapping) from
       information that changes less frequently (the list of virtual domain
       names).

       Specify a list of host or domain names, "/file/name" or "type:table"
       patterns, separated by commas and/or whitespace. A "/file/name" pattern
       is replaced by its contents; a "type:table" lookup table is matched
       when a table entry matches a host or domain name (the lookup result is
       ignored).  Continue long lines by starting the next line with
       whitespace. Specify "!pattern" to exclude a host or domain name from
       the list. The form "!/file/name" is supported only in Postfix version
       2.4 and later.

       See also the VIRTUAL_README and ADDRESS_CLASS_README documents for
       further information.

       Example:

       virtual_alias_domains = virtual1.tld virtual2.tld

virtual_alias_expansion_limit (default: 1000)
       The maximal number of addresses that virtual alias expansion produces
       from each original recipient.

       This feature is available in Postfix 2.1 and later.

virtual_alias_maps (default: $virtual_maps)
       Optional lookup tables that alias specific mail addresses or domains to
       other local or remote addresses.  The table format and lookups are
       documented in virtual(5). For an overview of Postfix address
       manipulations see the ADDRESS_REWRITING_README document.

       This feature is available in Postfix 2.0 and later. The default value
       is backwards compatible with Postfix version 1.1.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.  Note: these lookups are recursive.

       If you use this feature with indexed files, run "postmap
       /etc/postfix/virtual" after changing the file.

       Examples:

       virtual_alias_maps = dbm:/etc/postfix/virtual
       virtual_alias_maps = hash:/etc/postfix/virtual

virtual_alias_recursion_limit (default: 1000)
       The maximal nesting depth of virtual alias expansion.  Currently the
       recursion limit is applied only to the left branch of the expansion
       graph, so the depth of the tree can in the worst case reach the sum of
       the expansion and recursion limits.  This may change in the future.

       This feature is available in Postfix 2.1 and later.

virtual_delivery_status_filter (default: $default_delivery_status_filter)
       Optional filter for the virtual(8) delivery agent to change the
       delivery status code or explanatory text of successful or unsuccessful
       deliveries.  See default_delivery_status_filter for details.

       This feature is available in Postfix 3.0 and later.

virtual_destination_concurrency_limit (default:
       $default_destination_concurrency_limit)
       The maximal number of parallel deliveries to the same destination via
       the virtual message delivery transport. This limit is enforced by the
       queue manager. The message delivery transport name is the first field
       in the entry in the master.cf file.

virtual_destination_recipient_limit (default:
       $default_destination_recipient_limit)
       The maximal number of recipients per message for the virtual message
       delivery transport. This limit is enforced by the queue manager. The
       message delivery transport name is the first field in the entry in the
       master.cf file.

       Setting this parameter to a value of 1 changes the meaning of
       virtual_destination_concurrency_limit from concurrency per domain into
       concurrency per recipient.

virtual_gid_maps (default: empty)
       Lookup tables with the per-recipient group ID for virtual(8) mailbox
       delivery.

       This parameter is specific to the virtual(8) delivery agent.  It does
       not apply when mail is delivered with a different mail delivery
       program.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       In a lookup table, specify a left-hand side of "@domain.tld" to match
       any user in the specified domain that does not have a specific
       "user@domain.tld" entry.

       When a recipient address has an optional address extension
       (user+foo@domain.tld), the virtual(8) delivery agent looks up the full
       address first, and when the lookup fails, it looks up the unextended
       address (user@domain.tld).

       Note 1: for security reasons, the virtual(8) delivery agent disallows
       regular expression substitution of $1 etc. in regular expression lookup
       tables, because that would open a security hole.

       Note 2: for security reasons, the virtual(8) delivery agent will
       silently ignore requests to use the proxymap(8) server. Instead it will
       open the table directly. Before Postfix version 2.2, the virtual(8)
       delivery agent will terminate with a fatal error.

virtual_mailbox_base (default: empty)
       A prefix that the virtual(8) delivery agent prepends to all pathname
       results from $virtual_mailbox_maps table lookups.  This is a safety
       measure to ensure that an out of control map doesn't litter the file
       system with mailboxes.  While virtual_mailbox_base could be set to "/",
       this setting isn't recommended.

       This parameter is specific to the virtual(8) delivery agent.  It does
       not apply when mail is delivered with a different mail delivery
       program.

       Example:

       virtual_mailbox_base = /var/mail

virtual_mailbox_domains (default: $virtual_mailbox_maps)
       Postfix is the final destination for the specified list of domains;
       mail is delivered via the $virtual_transport mail delivery transport.
       By default this is the Postfix virtual(8) delivery agent.  The SMTP
       server validates recipient addresses with $virtual_mailbox_maps and
       rejects mail for non-existent recipients.  See also the virtual mailbox
       domain class in the ADDRESS_CLASS_README file.

       This parameter expects the same syntax as the mydestination
       configuration parameter.

       This feature is available in Postfix 2.0 and later. The default value
       is backwards compatible with Postfix version 1.1.

virtual_mailbox_limit (default: 51200000)
       The maximal size in bytes of an individual virtual(8) mailbox or
       maildir file, or zero (no limit).

       This parameter is specific to the virtual(8) delivery agent.  It does
       not apply when mail is delivered with a different mail delivery
       program.

virtual_mailbox_lock (default: see postconf -d output)
       How to lock a UNIX-style virtual(8) mailbox before attempting delivery.
       For a list of available file locking methods, use the "postconf -l"
       command.

       This parameter is specific to the virtual(8) delivery agent.  It does
       not apply when mail is delivered with a different mail delivery
       program.

       This setting is ignored with maildir style delivery, because such
       deliveries are safe without application-level locks.

       Note 1: the dotlock method requires that the recipient UID or GID has
       write access to the parent directory of the recipient's mailbox file.

       Note 2: the default setting of this parameter is system dependent.

virtual_mailbox_maps (default: empty)
       Optional lookup tables with all valid addresses in the domains that
       match $virtual_mailbox_domains.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       In a lookup table, specify a left-hand side of "@domain.tld" to match
       any user in the specified domain that does not have a specific
       "user@domain.tld" entry.

       With the default "virtual_mailbox_domains = $virtual_mailbox_maps",
       lookup tables also need entries with a left-hand side of "domain.tld"
       to satisfy virtual_mailbox_domain lookups (the right-hand side is
       required but will not be used).

       The remainder of this text is specific to the virtual(8) delivery
       agent.  It does not apply when mail is delivered with a different mail
       delivery program.

       The virtual(8) delivery agent uses this table to look up the
       per-recipient mailbox or maildir pathname.  If the lookup result ends
       in a slash ("/"), maildir-style delivery is carried out, otherwise the
       path is assumed to specify a UNIX-style mailbox file.  Note that
       $virtual_mailbox_base is unconditionally prepended to this path.

       When a recipient address has an optional address extension
       (user+foo@domain.tld), the virtual(8) delivery agent looks up the full
       address first, and when the lookup fails, it looks up the unextended
       address (user@domain.tld).

       Note 1: for security reasons, the virtual(8) delivery agent disallows
       regular expression substitution of $1 etc. in regular expression lookup
       tables, because that would open a security hole.

       Note 2: for security reasons, the virtual(8) delivery agent will
       silently ignore requests to use the proxymap(8) server. Instead it will
       open the table directly. Before Postfix version 2.2, the virtual(8)
       delivery agent will terminate with a fatal error.

virtual_maps (default: empty)
       Optional lookup tables with a) names of domains for which all addresses
       are aliased to addresses in other local or remote domains, and b)
       addresses that are aliased to addresses in other local or remote
       domains.  Available before Postfix version 2.0. With Postfix version
       2.0 and later, this is replaced by separate controls:
       virtual_alias_domains and virtual_alias_maps.

virtual_minimum_uid (default: 100)
       The minimum user ID value that the virtual(8) delivery agent accepts as
       a result from $virtual_uid_maps table lookup.  Returned values less
       than this will be rejected, and the message will be deferred.

       This parameter is specific to the virtual(8) delivery agent.  It does
       not apply when mail is delivered with a different mail delivery
       program.

virtual_transport (default: virtual)
       The default mail delivery transport and next-hop destination for final
       delivery to domains listed with $virtual_mailbox_domains.  This
       information can be overruled with the transport(5) table.

       Specify a string of the form transport:nexthop, where transport is the
       name of a mail delivery transport defined in master.cf.  The :nexthop
       destination is optional; its syntax is documented in the manual page of
       the corresponding delivery agent.

       This feature is available in Postfix 2.0 and later.

virtual_uid_maps (default: empty)
       Lookup tables with the per-recipient user ID that the virtual(8)
       delivery agent uses while writing to the recipient's mailbox.

       This parameter is specific to the virtual(8) delivery agent.  It does
       not apply when mail is delivered with a different mail delivery
       program.

       Specify zero or more "type:name" lookup tables, separated by whitespace
       or comma. Tables will be searched in the specified order until a match
       is found.

       In a lookup table, specify a left-hand side of "@domain.tld" to match
       any user in the specified domain that does not have a specific
       "user@domain.tld" entry.

       When a recipient address has an optional address extension
       (user+foo@domain.tld), the virtual(8) delivery agent looks up the full
       address first, and when the lookup fails, it looks up the unextended
       address (user@domain.tld).

       Note 1: for security reasons, the virtual(8) delivery agent disallows
       regular expression substitution of $1 etc. in regular expression lookup
       tables, because that would open a security hole.

       Note 2: for security reasons, the virtual(8) delivery agent will
       silently ignore requests to use the proxymap(8) server. Instead it will
       open the table directly. Before Postfix version 2.2, the virtual(8)
       delivery agent will terminate with a fatal error.

SEE ALSO
       postconf(1), Postfix configuration parameter maintenance
       master(5), Postfix daemon configuration maintenance

LICENSE
       The Secure Mailer license must be distributed with this software.

AUTHOR(S)
       Wietse Venema
       IBM T.J. Watson Research
       P.O. Box 704
       Yorktown Heights, NY 10598, USA

       Wietse Venema
       Google, Inc.
       111 8th Avenue
       New York, NY 10011, USA

       Viktor Dukhovni



                                                                   POSTCONF(5)