What is Postfix, Postfixadmin, Cyrus-SASL, Courier Authlib, Courier-Imap, Squirellmail ?

Postfix is a free open source mail transfer agent (MTA), a computer program for the routing and delivery of email. It is intended as a fast, easy-to-administer, and secure alternative to the widely-used Sendmail MTA.

Postfix Admin is a web based interface to configure and manage a Postfix based email server for many users.