Mailtron Gateway is a POP (and for Mac OS X, a POP/IMAP) email gateway from Studiotron Software that allows multiple people at a single domain to share a single email account. The license key that is required after the initial 30-day usage period is free for "Classic" Mac OS. Version 3.x (for Mac OS X only), however, comes as a fully-functional demo that runs for 30 days (except that he fetchmail daemon will not automatically run after a system restart) and can be registered for $69. The Mac OS X version features "industrial strength" mail forwarding using fetchmail, and automatic protocol detection, including IMAP.
Version 3.0 - the latest release for Mac OS X - adds/addresses the following:
"This fine, free program works as advertised. It is easy to set up and runs like a champ. We use it to route all the Internet email for our entire company (15 users)."
—Rob Fiorendino
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Also, if you have an older Mac, be sure to check out the "Classic" applications page for more options.
Finally, take a look at ALEMIA if you think you know that name of an application, but aren't quite sure.
Mac OS X has a huge amount of TCP/IP-based server software built into it that I don't specifically cover here. Your "Sharing" Preference Pane allows you to enable and disable these services with a click of the mouse. The software running behind the scenes to provide many of these services is generally of the open source variety. The standard release of Mac OS X includes, among many others:
Of course, Mac OS X Server includes many more, in addition to offering more recent versions of many of the above servers.
Graham Orndorff has written a superb collection of articles on setting up email servers and secure email clients on Mac OS X.
These are applications that are newer and of potential interest, but which I haven't yet selected for permanent inclusion. Have a look, and let me know if you think they deserve to be part of the permanent collection!