Open source email server for Unix
Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail. It is intended as a fast, easier-to-administer, and secure alternative to the widely-used Sendmail MTA.
The Postfix Home Page: Home page for Postfix, contains up to date documentation, Howtos and FAQs, and a great deal more useful information.
Ubuntu Community Help Wiki - Postfix: Very detailed Ubuntu-centric information on the setup of Postfix.