The queue manager daemon qmgr is in many ways the heart of your Postfix system. All messages, both outbound and inbound, must pass through the queue. It's a good idea to understand the queue and how Postfix uses it in case you have to troubleshoot a problem.
The queue manager maintains five different queues: incoming, active, deferred, hold, and corrupt.
Postfix uses a separate directory for each queue below the path specified
queue_directory parameter. By default the path is /var/spool/postfix, which gives you a directory
structure like the following:
/var/spool/postfix/active /var/spool/postfix/bounce /var/spool/postfix/corrupt /var/spool/postfix/deferred /var/spool/postfix/hold
The qmgr daemon running in the background handles most of the queue management tasks automatically. The commands postsuper and postqueue are used by administrators for manual queue management tasks. This chapter looks at how qmgr and the command-line tools work, as well as Postfix parameters that affect the queue.
Figure 5-1 illustrates how messages move through the queue. The
incoming queue is where messages first enter Postfix. The queue manager
provides protection for the queue filesystem through the
queue_minfree parameter. The default value is 0. You can make sure the
disk that stores your queue doesn't run out of space by setting a
Figure 5-1. Message ...