Postfix is an attempt to provide an easy to use, fast and secure alternative MTA to the widely-used Sendmail program. Despite Sendmail’s wide use, Postfix is the default MTA for a number of os distributions, including Ubuntu.
Instructions for setting up on Ubuntu, and here for Centos.
Postfix is licensed under a GPL incompatible license: the IBM public license. This means that Postfix cannot be linked with GPL code.