The Courier mail transfer agent (MTA) is a modular multiprotocol mail
server that's designed to strike a balance between reasonable performance,
flexibility and features.

When Courier-MTA is installed together with Courier-IMAP and SqWebMail,
they form an integrated mail/groupware server suite that provides
ESMTP, IMAP, POP3, webmail, and mailing list services within a single,
consistent framework.  A web-based administration and configuration
tool is included for comprehensive configuration of the entire Courier
software suite.

Homepage:
http://www.courier-mta.org/
