Qmail

From Grokdoc

qmail is a fast, lightweight, and very secure email server (MTA) developed by D. J. Berstein. Its primary focus is security. It has seperation of priveledges, and all the code comes under a $500 guarantee, which has not been claimed yet. The last change was in 1998 which some feel as a bad thing, others believe that because it has not been changed, that it contains everything that is needed in a MTA.

qmail is normally installed with daemontools, which contains tcpserver which acts as the daemon that talks to the programs that come with qmail (pop3d, and smtpd)

Adding functionality to qmail is done by patching it, most patches that are released can be found on qmail.org. Patching is the only way that you can add functionality, because releasing a modified qmail tarball is not allowed.

Allthough there are places like qmailrocks.org the suggested install method on any system and distrobution is always lifewithqmail.org as this will give you the greatest amount of knowledge, and you will know what to do on your own, rather than have some script do it. It will take longer, but you will understand exactly how qmail works, and where everything is if it goes wrong.

Qmailrocks.org is a place to get help installing this MTA on a variety of platforms, as well as a variety of other tools such as SpamAssassin(Spam Blocking) and ClamAV(Antivirus), do not expect a lot help from the qmail list if you use this tool, as they will point you in the direction of lifewithqmail.org

Please note that it comes with non-free licence terms and motivations.