The previous sections gave a simplified overview of how the Postfix
system sends and receives mail. Several other things happen behind
the scenes. Unfortunately, this is hard to visualize on a
two-dimensional display, so this document has no illustration.
The master daemon is the supervisor
process that keeps an eye on the well-being of the mail system. It
is typically started at system boot time by the postfix command, and keeps running until
the system goes down. The master daemon
is responsible for starting all other Postfix daemon processes on
demand, and for restarting daemons that terminated prematurely
because of some problem. The master
daemon is also responsible for enforcing the daemon process count
limits as specified in the master.cf configuration file.
The bounce or defer daemon is called
upon left and right by other daemon processes, in order to maintain
per-message log files with non-delivery status information.
The trivial-rewrite daemon
is called upon left and right by other daemon processes, in order
to rewrite an address to email@example.com form,
or in order to resolve a destination.
The showq daemon lists the Postfix
queue status. This is the program behind the mailq command.