Postfix/flow

About

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 (the "-<" between the last two boxes in the original document is apparently intended to indicate rightward movement with a fan-out, i.e. the message may go through any of the 3 sub-boxes on the right; it has been replaced with "->" here).

		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
		^ |				^ | | 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. There are two more diagrams here, showing where filtering takes place.