Notes on the Postfix mail server

This is not a step-to-step guide to set up Postfix, but rather a collection of notes that will help you understand how Postfix and e-mail works. It is still work in progress. Postfix is complex; why? because e-mail is complex due to its long-term decentralized evolution. To cut down on the refactoring cost for the developer, Postfix is designed as a set of programs, each having a specific duty. To many it may seem a peculiar design compared to other programs, and there's certainly alternatives to Postfix.

A high-level overview of how Postfix works is presented here. I recommend a cursory reading of RFC 821 [1], which while obsoleted by subsequent RFCs, should contain a lot of illuminating information on how e-mail operates. In particular, keep in mind that SMTP is a server-to-server protocol, and no clients (e.g. like thunderbird) are involved.

A (simplified) description of how e-mail travels through Postfix is this:

Received e-mail delivery (processing) can be understood in terms of filtering and delivery.

Filtering can occur in smtpd(8), cleanup(8), and in the delivery agents.

The processing by local(8) and virtual(8) is for storage as files or to be processed further by a user command. The pipe(8) agent also passes the e-mail content to a supplied command's stdin, the difference being that local(8) is per Unix user, while pipe(8) is a lot more configurable and can e.g. run on a single user that is responsible for the processing (e.g. SpamAssassin)

There's a lot of processes associated with Postfix. The supervisor is called master(8).

Undoubtedly at some point you'd like to test your configuration by sending e-mail locally. Swaks is a handy tool for that purpose.