Qmail

QMail is a package to build a mail server. It is developed by Dan Bernstein.

Advantages of QMail: it is secure, fast, compact, incredibly configurable and portable, ...

Disadvantages: it needs to be build from source, needs patches to do special things, documentation is for geeks, imposes other goodies (like ucspi-tcp and daemontools) on the installer, ...

QMail is used by many exciting packages, such as EZ-MLM.