What is Postfix?

Postfix is an open-source mail transfer agent (MTA). It is intended as a fast, easy-to-administer, and secure alternative to the widely-used Sendmail. It is also extensible, allowing configuration and account data to be stored in databases, or in an LDAP directory.

What's so great about Postfix?

One of the strengths of Postfix is its resilience against buffer overflows. Another one is its handling of large amounts of e-mail. Postfix is built as a cooperating network of different daemons. Each daemon fulfills a single task using minimum privileges. In this way, if a daemon is compromised, the impact remains limited to that daemon and cannot spread throughout the entire system. There is only one process with root privileges (master), and a few (local, virtual, pipe) that actually write to disk or invoke external programs.

Postfix is the default MTA shipped with Mac OSX, as well as many Linux distributions.

There are many addons available for Postfix, including virus scanners, webmail clients, hosting control panels, and more. There's a Wiki available here.