Exim is a highly configurable and full-featured mail server. It is an open source mail transfer agent which is a program responsible for receiving, routing, and delivering e-mail messages. The configuration file provides a wide range of built-in options.

Postfix is a product of IBM research. Postfix is fast, secure, easy administer mail server that considers as an alternative to the sendmail. Built from source code, Postfix can run on UNIX-like systems including AIX, BSD, HP-UX, Linux, MacOS X, Solaris and more.

Qmail is supported on Unix and is a popular mail transfer agent as its secure and reliable. It uses multiple server processes and programs each of which only has the privileges that it needs whereas sendmail has a single server process that runs as root.

Microsoft Exchange Server (MXS) is a collaborative enterprise server application designed by Microsoft to run on Windows Servers. MXS supports Email, Contacts and tasks, Calendar, Web-based and mobile information access and Data storage