Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.