Postfix

Postfix is Wietse Venema’s mail server that started life at IBM research as an alternative to the widely-used Sendmail program.
Postfix attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different.
Built from source code, Postfix can run on UNIX-like systems including AIX, BSD, HP-UX, Linux, MacOS X, Solaris, and more.
Postfix is also distributed as ready-to-run code by operating system vendors, appliance vendors, and other providers. Their versions may have small differences with the software that is described on Postfix website.

Source: http://www.postfix.org/