Postfix/qmail are cases where you have more than one process doing only one task and good. Like for instance you may want to deliver some mails locally, plug yo

Postfix/qmail are cases where you have more than one process doing only one task and good. Like for instance you may want to deliver some mails locally, plug your own delivery system, or pass your mail through an anti spam/virus proxy from a 3rd party. You may even have process some to make custom authentications, routing ... For this you may prefer to use unix domain socket, or even (if you use maildir) just the give the name of the file to a local process and avoid the extra fat of network communications. And for scaling postfix prefers a multiprocessing approach to a multithreading approach. Sometimes you can communicate better as distinct processes than as distinct threads especially when you use the normal unix convention of using SIGHUP for reloading. (HINT, kill and threads are no friends)