Postfix

About

Postfix is an open-source mail transfer agent (MTA) for POSIX systems. It is fully supported by Webmin and is part of the Dovecot stack.

Files

• /main.cf
• /master.cf: formatted list of transport helper applications

Notes

After making changes to the Postfix configuration files, you don't actually have to stop and restart Postfix to load the changes; you can just type (as root) "postfix reload".

flow

There are multiple stages inside Postfix, and each stage can be affected by configuration parameters. I have not yet found a clear documentation of the data flow, but it seems to go like this (only much more complicated) under normal circumstances:

SMTP → Postfix delivery queue → user alias mappings → local delivery (MDA, e.g. Dovecot)

However, when using virtual (non-system) users, internally-generated emails (e.g. mail from cron or PHP's mail() function) seem to bypass the alias mappings, which the MDA then rejects if they are addressed to an alias instead of a user.

A table from the official documentation is more detailed, if perhaps less clear (where does the message end up? qmgr? how does it get delivered?):

		trivial- rewrite(8) (std form)						trivial- rewrite(8) (resolve)
		^ | | v						^ | | v
smtpd(8)	>-	cleanup(8)	->	incoming	->	active	->	qmgr(8)	-<	smtp(8)
qmqpd(8)										lmtp(8)
pickup(8)										local(8)
		^ |				^ | | v
		bounces forwarding notices				deferred

There are two more diagrams here; the first shows the input stage in a bit more detail, and the second shows the delivery stage.

aliases

To set up aliases, either edit the file specified by the alias_maps setting in main.cf (/etc/aliases by default) and then run newaliases (or "postalias /etc/aliases") to update the database, or else use Webmin ("Mail Aliases" icon, then click on the "Create a new alias" link).

Notes:

• Webmin seems to automatically run newaliases when you add or modify an individual alias, but not when you use it to edit the /etc/aliases file directly. Changes made to /etc/aliases will not take effect until the database is updated.
• Stopping and starting the postfix daemon does not update from the aliases file; you still have to run newaliases.

virtual users

By default, virtual aliases seem to get looked up only when email comes from the outside; locally generated emails are "returned" as undeliverable with the error message "Diagnostic-Code: x-unix; user unknown"

The following change in main.cf solves that problem –

• change: receive_override_options = no_address_mappings
• to this: receive_override_options =

– but creates a new one, in that externally-sourced messages are now delivered twice (or four times if addressed directly to a virtual user instead of a virtual alias).

Someone who understands the internal structure of postfix could probably figure out what this means (and how to fix it), but I'm still working on it.

catch-all

To set up a catch-all address, edit main.cf and set the following parameters:

• local_recipient_maps =
• luser_relay = whereToSend

The syntax for whereToSend is explained here.

Examples:

• luser_relay = $user@anotherdomain.org
  • forwards all email addressed to "invaliduser@firstdomain.org" to "invaliduser@anotherdomain.org"
• luser_relay = youraddress@anotherdomain.org
  • forwards all email addressed to invalid users to "youraddress@anotherdomain.org".

automatic BCC

To automatically BCC outgoing mail from any given set of users, set sender_bcc_maps in /main.cf to hash:/etc/postfix/sender_bcc. (In Webmin: BCC Mapping: Sender BCC mapping lookup tables)

Each line in the /sender_bcc file will contain the sender (which can be user@domain, user, or @domain), one or more blanks, then the recipient (which can be one address or multiple addresses separated by commas).

Example -- BCCs all mail sent by vbz.com users to an archive address:

@vbz.com archive2024@vbz.com

pages to index

I have these open in tabs, so they are probably relevant to something:

• Postfix Address Rewriting
• trivial-rewrite (manpage)
• Postfix Configuration Parameters (includes /main.cf parameters)

Links

Official

• home page

Reference

• Wikipedia
• wiki
  • Relay recipient maps using LDAP against Active Directory