Rejecting Unknown Local Recipients with Postfix

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Introduction

As of Postfix version 2.0, the Postfix SMTP server rejects mail for unknown
recipients in local domains (domains that match $mydestination or the IP
addresses in $inet_interfaces or $proxy_interfaces) with "User unknown in local
recipient table". This feature was optional with earlier Postfix versions.

The good news is that this keeps undeliverable mail out of your queue, so that
your mail queue is not clogged up with undeliverable MAILER-DAEMON messages.

The bad news is that it may cause mail to be rejected when you upgrade from a
Postfix system that was not configured to reject mail for unknown local
recipients.

This document describes what steps are needed in order to reject unknown local
recipients correctly.

  * Configuring local_recipient_maps in main.cf
  * When you need to change the local_recipient_maps setting in main.cf
  * Local recipient table format

Configuring local_recipient_maps in main.cf

The local_recipient_maps parameter specifies 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. If a local
username or address is not listed in $local_recipient_maps, then the Postfix
SMTP server will reject the address with "User unknown in local recipient
table".

The default setting, shown below, assumes that you use the default Postfix
local(8) delivery agent for local delivery, where recipients are either UNIX
accounts or local aliases:

    /etc/postfix/main.cf:
        local_recipient_maps = proxy:unix:passwd.byname $alias_maps

To turn off unknown local recipient rejects by the SMTP server, specify:

    /etc/postfix/main.cf:
        local_recipient_maps =

That is, an empty value. With this setting, the Postfix SMTP server will not
reject mail with "User unknown in local recipient table". Don't do this on
systems that receive mail directly from the Internet. With today's worms and
viruses, Postfix will become a backscatter source: it accepts mail for non-
existent recipients and then tries to return that mail as "undeliverable" to
the often forged sender address.

When you need to change the local_recipient_maps setting in main.cf

  * Problem: you don't use the default Postfix local(8) delivery agent for
    domains matching $mydestination, $inet_interfaces, or $proxy_interfaces.
    For example, you redefined the "local_transport" setting in main.cf.

    Solution: your local_recipient_maps setting needs to specify a database
    that lists all the known user names or addresses for that delivery agent.
    For example, if you deliver users in $mydestination etc. domains via the
    virtual(8) delivery agent, specify:

    /etc/postfix/main.cf
        mydestination = $myhostname localhost.$mydomain localhost ...
        local_transport = virtual
        local_recipient_maps = $virtual_mailbox_maps

    If you use a different delivery agent for $mydestination etc. domains, see
    the section "Local recipient table format" below for a description of how
    the table should be populated.

  * Problem: you use the mailbox_transport or fallback_transport feature of the
    Postfix local(8) delivery agent in order to deliver mail to non-UNIX
    accounts.

    Solution: you need to add the database that lists the non-UNIX users:

    /etc/postfix/main.cf
        local_recipient_maps = proxy:unix:passwd.byname, $alias_maps,
            <the database with non-UNIX accounts>

    See the section "Local recipient table format" below for a description of
    how the table should be populated.

  * Problem: you use the luser_relay feature of the Postfix local delivery
    agent.

    Solution: you must disable the local_recipient_maps feature completely, so
    that Postfix accepts mail for all local addresses:

    /etc/postfix/main.cf
        local_recipient_maps =

Local recipient table format

If you use local files in postmap(1) format, then local_recipient_maps expects
the following table format:

  * In the left-hand side, specify a bare username, an "@domain.tld" wild-card,
    or specify a complete "user@domain.tld" address.

  * You have to specify something on the right-hand side of the table, but the
    value is ignored by local_recipient_maps.

If you use lookup tables based on NIS, LDAP, MYSQL, or PGSQL, then
local_recipient_maps does the same queries as for local files in postmap(1)
format, and expects the same results.

With regular expression tables, Postfix only queries with the full recipient
address, and not with the bare username or the "@domain.tld" wild-card.

NOTE: a lookup table should always return a result when the address exists, and
should always return "not found" when the address does not exist. In
particular, a zero-length result does not count as a "not found" result.

