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.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
HEY - Email at its best, new from Basecamp.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.