Exim is 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, including database lookups and easy integration with SpamAssassin, ClamAV, and other spam and virus scanning programs.
Postfix is a product of IBM research. Postfix is fast, secure, easy administer mail server that consider 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.
Microsoft Exchange Server
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
Sendmail is popular mail transfer agent used today. It has many useful features for routing and processing email, such as aliases, domain routing and user-creatable forward files. It has a one-to-one mapping between Unix users and mailboxes.
MDaemon Messaging Server supports IMAP, SMTP, and POP3 protocols and delivers solid performance from its feature-rich and user-friendly design.It supports mailing lists, content filtering, multiple domains while offering flexible administration and an open standards design.
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 single server process that run as root.