Postfix

Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail. It attempts to be fast, easy to administer, and secure. The software is also known by its former names VMailer and IBM Secure Mailer.

Free Open Source Mac OS X Linux

Exim

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.

Free Open Source Mac OS X Windows Linux

Mail-in-a-box

Take back control of your email with this easy-to-deploy mail server in a box. Mail-in-a-Box turns a fresh cloud computer into a working mail server. You get contact synchronization, spam filtering, and so on.

Free Open Source Self-Hosted Roundcube ownCloud

Sendmail

Sendmail is a general purpose internetwork email routing facility that supports many kinds of transfer and delivery methods, including the Simple Mail Transfer Protocol (SMTP) used for email transport over the Internet.

Free Open Source Mac OS X Linux

OpenSMTPD

OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with some additional standard extensions. It allows ordinary machines to...

Free Open Source Linux BSD Self-Hosted