iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
Roundcube - Web-based IMAP email client