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Postfix + Maildrop Howto

Introduction

This document discusses various options to plug the maildrop delivery agent into Postfix:

Direct delivery without the local delivery agent
Indirect delivery via the local delivery agent
Credits

Direct delivery without the local delivery agent

Postfix can be configured to deliver mail directly to maildrop, without using the local(8) delivery agent as an intermediate. This means that you do not get local aliases(5) expansion or $HOME/.forward file processing. You would typically do this for hosted domains with recipients that don't have UNIX home directories.

The following example shows how to use maildrop for some.domain and for someother.domain.

 1 /usr/local/etc/postfix/main.cf:
 2     maildrop_destination_recipient_limit = 1
 3     virtual_mailbox_domains = some.domain someother.domain
 4     virtual_transport = maildrop
 5     virtual_mailbox_maps = hash:/usr/local/etc/postfix/virtual_mailbox
 6     virtual_alias_maps = hash:/usr/local/etc/postfix/virtual_alias
 7 
 8 /usr/local/etc/postfix/virtual_mailbox:
 9     user1@some.domain        ...text here does not matter...
10     user2@some.domain        ...text here does not matter...
11     user3@someother.domain   ...text here does not matter...
12 
13 /usr/local/etc/postfix/virtual_alias:
14     postmaster@some.domain           postmaster
15     postmaster@someother.domain      postmaster

Line 2 is needed so that Postfix will provide one recipient at a time to the maildrop delivery agent.
Line 3 informs Postfix that some.domain and someother.domain are so-called virtual mailbox domains. Instead of listing the names in main.cf you can also list them in a file; see the virtual_mailbox_domains documentation for details.
Line 4 specifies that mail for some.domain and someother.domain should be delivered by the maildrop delivery agent.
Lines 5 and 8-11 specify what recipients the Postfix SMTP server should receive mail for. This prevents the mail queue from becoming clogged with undeliverable messages. Specify an empty value ("virtual_mailbox_maps =") to disable this feature.
Lines 6 and 13-15 redirect mail for postmaster to the local postmaster. RFC 821 requires that every domain has a postmaster address.

The vmail userid as used below is the user that maildrop should run as. This would be the owner of the virtual mailboxes if they all have the same owner. If maildrop is suid (see maildrop documentation), then maildrop will change to the appropriate owner to deliver the mail.

Note: Do not use the postfix user as the maildrop user.

/usr/local/etc/postfix/master.cf:
    maildrop  unix  -       n       n       -       -       pipe
      flags=DRhu user=vmail argv=/path/to/maildrop -d ${recipient}

If you want to support user+extension@domain style addresses, use the following instead:

/usr/local/etc/postfix/master.cf:
    maildrop  unix  -       n       n       -       -       pipe
      flags=DRhu user=vmail argv=/path/to/maildrop 
      -d ${user}@${nexthop} ${extension} ${recipient} ${user} ${nexthop}

The mail is delivered to ${user}@${nexthop} (match key for maildrop userdb lookup). The ${extension} and the other address components are available to maildrop rules as $1, $2, $3, ... and can be omitted from master.cf or ignored by maildrop when not needed.

Indirect delivery via the local delivery agent

Postfix can be configured to deliver mail to maildrop via the local delivery agent. This is slightly less efficient than the "direct" approach discussed above, but gives you the convenience of local aliases(5) expansion and $HOME/.forward file processing. You would typically use this for domains that are listed in mydestination and that have users with a UNIX system account.

To configure maildrop delivery for all UNIX system accounts:

/usr/local/etc/postfix/main.cf:
    mailbox_command = /path/to/maildrop -d ${USER}

Note: ${USER} is spelled in upper case.

To enable maildrop delivery for specific users only, you can use the Postfix local(8) delivery agent's mailbox_command_maps feature:

/usr/local/etc/postfix/main.cf:
    mailbox_command_maps = /usr/local/etc/postfix/mailbox_commands

/usr/local/etc/postfix/mailbox_commands:
    you    /path/to/maildrop -d ${USER}

Maildrop delivery for specific users is also possible by invoking it from the user's $HOME/.forward file:

/home/you/.forward:
    "|/path/to/maildrop -d ${USER}"

Credits

The original text was kindly provided by Russell Mosemann.
Victor Duchovni provided tips for supporting user+foo@domain addresses.
Tonni Earnshaw contributed text about delivery via the local(8) delivery agent.