Postfix Anatomy - Receiving Mail

When a message enters the Postfix mail system, the first stop on the inside is the incoming queue. The figure below shows the main components that are involved with new mail. For an explanation of the symbols used, click on the icon in the upper left-hand corner of this page.

• Mail is posted locally. The Postfix sendmail program deposits the message into the world-writable maildrop directory, where the message is picked up by the pickup daemon. This daemon does some sanity checks, in order to protect the rest of the Postfix system. In order to avoid accidents, the directory permissions on the maildrop directory must be such that a user cannot delete someone elses mail.

• Mail comes in via the network. The Postfix SMTP server receives the message and does some sanity checks, in order to protect the rest of the Postfix system. The SMTP server can be configured to implement UCE controls on the basis of local or network-based black lists, DNS lookups, and other client request information.

• Mail is generated internally by the Postfix system itself, in order to return undeliverable mail to the sender. The bounce or defer daemon brings the bad news.

• Mail is forwarded by the local delivery agent, either via an entry in the system-wide alias database, or via an entry in a per-user .forward file. This is indicated with the unlabeled arrow.

• Mail is generated internally by the Postfix system itself, in order to notify the postmaster of a problem (this path is also indicated with the unlabeled arrow). The Postfix system can be configured to notify the postmaster of SMTP protocol problems, UCE policy violations, and so on.

• The cleanup daemon implements the final processing stage for new mail. It adds missing From: and other message headers, arranges for address rewriting to the standard user@fully.qualified.domain form, and optionally extracts recipient addresses from message headers. The cleanup daemon inserts the result as a single queue file into the incoming queue, and notifies the queue manager of the arrival of new mail. The cleanup daemon can be configured to transform addresses on the basis of canonical and virtual table lookups.

• On request by the cleanup daemon, the trivial-rewrite daemon rewrites addresses to the standard user@fully.qualified.domain form. The initial Postfix version does not implement a rewriting language. Implementing one would take a lot of effort, and most sites do not need it. Instead, Postfix makes extensive use of table lookup.