# Postfix Configuration - UCE Controls

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## Introduction

Postfix offers a variety of parameters that limit the delivery of unsolicited commercial email (UCE).

By default, the Postfix SMTP server will accept mail only from or to the local network or domain, or to domains that are hosted by Postfix, so that your system can't be used as a mail relay to forward bulk mail from random strangers.

The text in this document describes how you can set up more detailed anti-UCE policies that prevent delivery of unwanted email altogether, for example with sendmail-style access lists or with RBL (real-time blackhole list) name servers.

Unless indicated otherwise, all parameters described here are in the main.cf file. If you change parameters of a running Postfix system, don't forget to issue a postfix reload command.

- Header filtering

- Body filtering

- Client hostname/address restrictions

- Require HELO (EHLO) command

- HELO (EHLO) hostname restrictions

- Require strict RFC 821-style envelope addresses

- Sender address restrictions

- Recipient address restrictions

- ETRN command restrictions

- Generic restrictions

- Additional UCE control parameters

## Header filtering

The header_checks parameter restricts what is allowed in message headers. Patterns are applied to entire logical message headers, even when a header spans multiple lines of text.

Default:
  Allow anything in message headers.

Syntax:
  Specify a list of zero or more lookup tables. Whenever a header matches a table, the action depends on the lookup result:

  REJECT
    Reject the message, and log the header.
  REJECT text...
    As above, and also send the text to the originator.
  IGNORE
    Delete the header from the message.
  WARN
    Log (but do not reject) the header with a warning.
  WARN text...
    As above, and also log the text.

  At present, specifying a header pattern with OK serves no useful purpose. A rule ending in OK affects only the header being matched. The next header may still result in a REJECT match, causing the mail still to be rejected.

Examples (main.cf):
  header_checks = regexp:/etc/postfix/header_checks
  header_checks = pcre:/etc/postfix/header_checks

Example (header_checks):
  `/^to: *friend@public\.com$/ REJECT`

  ## Body filtering

  The body_checks parameter restricts what text is is allowed in message body lines (including MIME headers within the message body).

  Note: the message body is matched one line at a time. There is no multi-line concept as with message headers.

  Default:
    Allow anything in message body lines.

  Syntax:
    Specify a list of zero or more lookup tables. Whenever a body line matches a table, the action depends on the lookup result:

    REJECT
      Reject the message, and log the matched line.
    REJECT text...
      As above, and also send the text to the originator.
    IGNORE
      Delete the matched line from the message.
    WARN
      Log (but do not reject) the matched line with a warning.
    WARN text...
      As above, and also log the text.

    At present, specifying a pattern with OK serves no useful purpose. A rule ending in OK affects only the line being matched. The next line may still result in a REJECT match, causing the mail still to be rejected.

Examples (main.cf):
  body_checks = regexp:/etc/postfix/body_checks
  body_checks = pcre:/etc/postfix/body_checks

  ## Client hostname/address restrictions

  The smtpd_client_restrictions parameter restricts what clients this system accepts SMTP connections from.

  By default, this restriction is applied when the client sends the RCPT TO command. In order to have the restriction take effect as soon as possible, specify smtpd_delay_reject = no in the Postfix main.cf configuration file. Doing so may cause unexpected results with poorly implemented client software.

  Default:
    smtpd_client_restrictions =

    Allow SMTP connections from any client.

  Syntax:
    Specify a list of zero or more restrictions, separated by whitespace or commas. Restrictions are applied in the order as specified; the first restriction that matches wins.

  Examples:
    smtpd_client_restrictions = hash:/etc/postfix/access, reject_maps_rbl
    smtpd_client_restrictions = permit_mynetworks, reject_unknown_client

  Restrictions:

  reject_unknown_client
    Reject the request when the client IP address has no PTR record in the DNS. The unknown_client_reject_code parameter specifies the response code to rejected requests (default: 450).

  permit_mynetworks
    Permit the request when the client IP address matches any network listed in $mynetworks.

  check_client_access maptype:mapname
  maptype:mapname
    Search the named access database for the client hostname, parent domains, client IP address, or networks obtained by stripping least significant octets. Reject the request if the result is REJECT or "[45]XX text". Permit the request if the result is OK or RELAY or all-numerical. Otherwise, treat the result as another list of UCE restrictions. The access_map_reject_code parameter specifies the response code for REJECT results (default: 554).

  reject_maps_rbl
    Reject the request when the reversed client network address is listed under any of the domains listed in $maps_rbl_domains. The maps_rbl_reject_code parameter specifies the response code for rejected requests (default: 554).

  permit
  reject
  warn_if_reject
  reject_unauth_pipelining
    See generic restrictions.

  ## Require HELO (EHLO) command

  The smtpd_helo_required parameter determines if clients must send a HELO (or EHLO) command at the beginning of an SMTP session. Requiring this will stop some UCE software.

  Default:
    smtpd_helo_required = no

    By default, the Postfix SMTP server does not require the use of HELO (EHLO).

  Syntax:
    Specify yes or no.

  Example:
    smtpd_helo_required = yes

  ## HELO (EHLO) hostname restrictions

  The smtpd_helo_restrictions parameter restricts what hostnames clients may send with the HELO (EHLO) command. Some UCE software can be stopped by being strict here.

  By default, this restriction is applied when the client sends the RCPT TO command. In order to have the restriction take effect as soon as possible, specify smtpd_delay_reject = yes in the Postfix main.cf configuration file. Doing so may cause unexpected results with poorly implemented client software.

  Default:
    smtpd_helo_restrictions =

    By default, the Postfix SMTP server accepts any garbage in the HELO (EHLO) command. There is a lot of broken or misconfigured software on the Internet.

  Syntax:
    Specify a list of zero or more restrictions, separated by whitespace or commas. Restrictions are applied in the order as specified; the first restriction that matches wins.

    In addition to restrictions that are specific to HELO (EHLO) command parameters, you can also specify restrictions based on the client hostname or network address.

  Example:
    smtpd_helo_restrictions = permit_mynetworks, reject_invalid_hostname

  Restrictions:

  reject_invalid_hostname
    Reject the request when the client HELO or EHLO parameter has a bad hostname syntax. The invalid_hostname_reject_code specifies the response code to rejected requests (default: 501).

  permit_naked_ip_address
    Permit the request when the client HELO (EHLO) command contains a naked IP address without the enclosing [] brackets that the RFC requires. Unfortunately, some popular PC mail clients send HELO greetings in this manner.

  reject_unknown_hostname
    Reject the request when the hostname in the client HELO (EHLO) command has no DNS A or MX record. The unknown_hostname_reject_code specifies the response code to rejected requests (default: 450).

  reject_non_fqdn_hostname
    Reject the request when the hostname in the client HELO (EHLO) command is not in fully-qualified domain form, as required by the RFC. The non_fqdn_reject_code specifies the response code to rejected requests (default: 504).

  check_helo_access maptype:mapname
  maptype:mapname
    Search the named access database for the HELO hostname or parent domains in the specified table. Reject the request if the result is REJECT or "[45]XX text". Permit the request when the result is OK or RELAY or all-numerical. Otherwise, treat the result as another list of UCE restrictions. The access_map_reject_code parameter specifies the response code for REJECT results (default: 554).

  reject_maps_rbl
  reject_unknown_client
  permit_mynetworks
  check_client_access maptype:mapname
    See client hostname/address restrictions.

  permit
  reject
  warn_if_reject
  reject_unauth_pipelining
    See generic restrictions.

  ## Require strict RFC 821-style envelope addresses

  The strict_rfc821_envelopes parameter controls how tolerant Postfix is with respect to addresses given in MAIL FROM or RCPT TO commands. Unfortunately, the widely-used Sendmail program tolerates lots of non-standard behavior, so a lot of software expects to get away with it. Being strict to the RFC not only stops unwanted mail, it also blocks legitimate mail from poorly-written mail applications.

  Default:
    strict_rfc821_envelopes = no

    By default, the Postfix SMTP server accepts any address form that it can make sense of, including address forms that contain RFC 822-style comments, or addresses not enclosed in <>. There is a lot of broken or misconfigured software out there on the Internet.

  Example:
    strict_rfc821_envelopes = yes

  ## Sender address restrictions

  The smtpd_sender_restrictions parameter restricts what sender addresses this system accepts in MAIL FROM commands.

  By default, this restriction is applied when the client sends the RCPT TO command. In order to have the restriction take effect as soon as possible, specify smtpd_delay_reject = yes in the Postfix main.cf configuration file. Doing so may cause unexpected results with poorly implemented client software.

  Default:
    smtpd_sender_restrictions =

    By default, the Postfix SMTP server accepts any sender address.

  Syntax:
    Specify a list of zero or more restrictions, separated by whitespace or commas. Restrictions are applied in the order as specified; the first restriction that matches wins.

    In addition to restrictions that are specific to sender mail addresses, you can also specify restrictions based on the information passed with the HELO/EHLO command, and on the client hostname or network address.

  Example:
    smtpd_sender_restrictions = hash:/etc/postfix/access, reject_unknown_sender_domain

  Restrictions:
  reject_unknown_sender_domain
    Reject the request when the sender mail address has no DNS A or MX record. The unknown_address_reject_code parameter specifies the response code for rejected requests (default: 450). The response is always 450 in case of a temporary DNS error.

  check_sender_access maptype:mapname
  maptype:mapname
    Search the named access database for the sender mail address, parent domain, or localpart@. Reject the request if the result is REJECT or "[45]XX text". Permit the request if the result is OK or RELAY or all-numerical. Otherwise, treat the result as another list of UCE restrictions. The access_map_reject_code parameter specifies the result code for rejected requests (default: 554).

  reject_non_fqdn_sender
    Reject the request when the address in the client MAIL FROM command is not in fully-qualified domain form. The non_fqdn_reject_code specifies the response code to rejected requests (default: 504).

  reject_sender_login_mismatch
    Reject the request when $smtpd_sender_owner_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.

  permit_naked_ip_address
  reject_invalid_hostname
  reject_unknown_hostname
  reject_non_fqdn_hostname
  check_helo_access maptype:mapname
    See HELO (EHLO) hostname restrictions.

  reject_maps_rbl
  reject_unknown_client
  permit_mynetworks
  check_client_access maptype:mapname
    See client hostname/address restrictions.

  permit
  reject
  warn_if_reject
  reject_unauth_pipelining
    See generic restrictions.

  ## Recipient address restrictions

  The smtpd_recipient_restrictions parameter restricts what recipient addresses this system accepts in RCPT TO commands.
  Default:
    smtpd_recipient_restrictions = permit_mynetworks, check_relay_domains

    By default, the Postfix SMTP server relays mail:

    - from trusted clients whose IP address matches $mynetworks,
    - from trusted clients whose hostname matches $relay_domains or a subdomain thereof,
    - from untrusted clients to destinations that match $relay_domains or a subdomain thereof, except for addresses that contain sender-specified routing (user@elsewhere@domain).

    In addition to the above, the Postfix SMTP server by default accepts mail for which Postfix is the final destination:

    - to destinations that match $inet_interfaces,
    - to destinations that match $mydestination,
    - to destinations that match $virtual_maps.

  Syntax:
    Specify a list of zero or more restrictions, separated by whitespace or commas. Restrictions are applied in the order as specified; the first restriction that matches wins.

    In addition to restrictions that are specific to recipient mail addresses, you can also specify restrictions based on the sender mail address, on the information passed with the HELO/EHLO command, and on the client hostname or network address.

  Example:
    smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination

    Note: you must specify at least one of the following restrictions: reject, check_relay_domains or reject_unauth_destination. Postfix will refuse to receive mail otherwise.

  Restrictions:
  check_relay_domains
    Permit the request when one of the following is true:
    - the client hostname matches $relay_domains or a subdomain thereof,
    - the resolved destination address matches $relay_domains or a subdomain thereof, and the address contains no sender-specified routing (user@elsewhere@domain),
    - Postfix is the final destination: any destination that matches $mydestination, $inet_interfaces or $virtual_maps.
    Otherwise reject the request. The relay_domains_reject_code parameter specifies the response code for rejected requests (default: 554).

  permit_auth_destination
    Ignore the client hostname. Permit the request when one of the following is true:
    - the resolved destination address matches $relay_domains or a subdomain thereof, and the address contains no sender-specified routing (user@elsewhere@domain),
    - Postfix is the final destination: any destination that matches $mydestination, $inet_interfaces or $virtual_maps.
    Otherwise proceed with the next restriction.

  reject_unauth_destination
    Ignore the client hostname. Reject the request unless one of the following is true:
    - the resolved destination address matches $relay_domains or a subdomain thereof, and the address contains no sender-specified routing (user@elsewhere@domain),
    - Postfix is the final destination: any destination that matches $mydestination, $inet_interfaces or $virtual_maps.
    The relay_domains_reject_code parameter specifies the response code for rejected requests (default: 554).

  permit_mx_backup
    Permit the request when the local mail system is MX host for the resolved destination. This includes the case that the local mail system is the final destination. However, the SMTP server will not forward mail with addresses that have sender-specified routing information (example: user@elsewhere@domain),

    Use the optional permit_mx_backup_networks parameter to also require that the primary MX hosts match a list of network blocks.

    Relevant configuration parameters: permit_mx_backup_networks, $mydestination, $inet_interfaces.

  check_recipient_access maptype:mapname
  maptype:mapname
    Search the named access database for the resolved destination address, parent domain, or localpart@. Reject the request if the result is REJECT or "[45]XX text". Permit the request if the result is OK or RELAY or all-numerical. Otherwise, treat the result as another list of UCE restrictions. The access_map_reject_code parameter specifies the result code for rejected requests (default: 554).

  reject_unknown_recipient_domain
    Reject the request when the recipient mail address has no DNS A or MX record. The unknown_address_reject_code parameter specifies the response code for rejected requests (default: 450). The response is always 450 in case of a temporary DNS error.

  reject_non_fqdn_recipient
    Reject the request when the address in the client RCPT TO command is not in fully-qualified domain form. The non_fqdn_reject_code specifies the response code to rejected requests (default: 504).

  reject_unknown_sender_domain
  reject_non_fqdn_sender
  check_sender_access maptype:mapname
  reject_sender_login_mismatch
    See sender address restrictions.

  permit_naked_ip_address
  reject_invalid_hostname
  reject_unknown_hostname
  reject_non_fqdn_hostname
  check_helo_access maptype:mapname
    See HELO (EHLO) hostname restrictions.

  reject_maps_rbl
  reject_unknown_client
  permit_mynetworks
  check_client_access maptype:mapname
    See client hostname/address restrictions.

  permit
  reject
  warn_if_reject
  reject_unauth_pipelining
    See generic restrictions.

  ## ETRN command restrictions

  Not really an UCE restriction, the smtpd_etrn_restrictions parameter restricts what domains can be specified in ETRN commands, and what clients can issue ETRN commands.
  Default:
    smtpd_etrn_restrictions =

    By default, the Postfix SMTP server accepts any ETRN command from any client.

  Syntax:
    Specify a list of zero or more restrictions, separated by whitespace or commas. Restrictions are applied in the order as specified; the first restriction that matches wins.

    In addition to restrictions that are specific to ETRN domain names, you can also specify restrictions based on the information passed with the HELO/EHLO command, and on the client hostname or network address.

  Example:
    smtpd_etrn_restrictions = permit_mynetworks, hash:/etc/postfix/etrn_access, reject

  Restrictions:
  check_etrn_access maptype:mapname
  maptype:mapname
    Search the named access database for the domain specified in the ETRN command, or its parent domains. Reject the request if the result is REJECT or "[45]XX text". Permit the request if the result is OK or RELAY or all-numerical. Otherwise, treat the result as another list of UCE restrictions. The access_map_reject_code parameter specifies the result code for rejected requests (default: 554).

  permit_naked_ip_address
  reject_invalid_hostname
  reject_unknown_hostname
  check_helo_access maptype:mapname
    See HELO (EHLO) hostname restrictions.

  reject_maps_rbl
  reject_unknown_client
  permit_mynetworks
  check_client_access maptype:mapname
    See client hostname/address restrictions.

  permit
  reject
  warn_if_reject
  reject_unauth_pipelining
    See generic restrictions.

  ## Generic restrictions

  The following restrictions can use used for client hostnames or addresses, for HELO (EHLO) hostnames, for sender mail addresses and for recipient mail addresses.
  Restrictions:

  permit
    Permit the request. This restriction is useful at the end of a restriction list, to make the default policy explicit.

  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 to rejected requests (default: 554).

  warn_if_reject
    Change the meaning of the next restriction, so that it logs a warning instead of rejecting a request (look for logfile records that contain "reject_warning"). This is useful for testing new restrictions in a "live" environment without risking unnecessary loss of mail.

  reject_unauth_pipelining
    Reject the request when the client sends SMTP commands ahead of time without knowing that Postfix actually supports SMTP command pipelining. This stops mail from bulk mail software that improperly uses SMTP command pipelining to speed up deliveries.

  ## Additional UCE control parameters

  permit_mx_backup_networks
    Restrict the use of the permit_mx_backup relay control feature to destinations whose primary MX hosts match a list of network blocks.

    Default:
      permit_mx_backup_networks =

      That is, all networks are authorized by default.

    Syntax:
      Specify a list of network blocks in CIDR (network/mask) notation, for example:

        permit_mx_backup_networks = 168.100.0.0/16

      You can also specify the absolute pathname of a pattern file instead of listing the patterns in the main.cf file.

  maps_rbl_domains
    This parameter controls the behavior of the reject_maps_rbl restriction that can appear as part of a client hostname/address restriction list.

    Default:
      maps_rbl_domains = blackholes.mail-abuse.org

      Note: RBL lookups are disabled by default.

    Syntax:
      Zero or more DNS domains that blacklist client IP addresses. A host is blacklisted when its reversed IP address is listed as a subdomain under any of the domains listed in $maps_rbl_domains.

  relay_domains
    This parameter controls the behavior of the check_relay_domains, reject_unauth_destination and permit_auth_destination restrictions that can appear as part of a recipient address restriction list.

    Default:
      relay_domains = $mydestination

      By default, the Postfix SMTP server relays mail:

      - from trusted clients whose IP address matches $mynetworks,
      - from trusted clients whose hostname matches $relay_domains or a subdomain thereof,
      - from untrusted clients to destinations that match $relay_domains or a subdomain thereof, except for addresses that contain sender-specified routing (user@elsewhere@domain).

    Syntax:
      Specify zero or more domain names, /file/name patterns and/or type:name lookup tables, separated by whitespace and/or commas. A /file/name is replaced by its contents; type:name requests that table lookup is done instead of string comparison.

    A host or destination address matches $relay_domains when its name or parent domain matches any of the names, files or lookup tables listed in $relay_domains.

  smtpd_sender_login_maps
    This parameter specifies ownership of MAIL FROM addresses, as used by the reject_sender_login_mismatch sender address restriction.

    Default:
      smtpd_sender_login_maps =

    Syntax:
      Specify zero or more type:name lookup tables, separated by whitespace and/or commas. The maps are searched in the specified order. Regexp tables are allowed.

      Each map entry specifies a sender address and the login name that owns the address. The search order is:

      user@domain owner
        This form has the highest precedence.

      user owner
        This matches user@site when site is equal to $myorigin, when site is listed in $mydestination, or when it is listed in $inet_interfaces.

      @domain owner
        This matches every address in the specified domain, and has the lowest precedence.

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