Postfix: The Definitive Guide

Tracing a Message Through Postfix

Let’s follow a typical message through the Postfix system. Figure 3-4, Figure 3-5, and Figure 3-6 illustrate the process as the message goes from the originating system to a destination MTA, which, in turn, forwards it to the final MTA, where it is held until the user is ready to read it. In Figure 3-4, Helene ([e-mail]) wants to send a message to Frank ([e-mail]). Helene has an account on a system that runs Postfix. Her email client lets her compose the message, and then it calls the Postfix sendmail command to send it. The Postfix sendmail command receives the message from Helene’s email software and deposits it into the maildrop directory. The pickup daemon then retrieves the message, performs its sanity checks, and feeds the message to the cleanup daemon, which performs the final processing on the new message. If Helene’s email client did not include a From: address, or did not use a fully-qualified hostname in the address, cleanup makes the necessary fixes to the message.

Figure 3-4. Tracing message delivery 1

Once finished, cleanup places the message into the incoming queue and notifies the queue manager that a new message is ready to be delivered. If the queue manager is ready to process new messages, it moves the message into the active queue. Because this message is destined for a user on an outside system, the queue ...