Qmail

qmail - an intensely secure, stable, and high-performance SMTP server. First server to use maildir instead of mbox message storage format.

See Mail toaster for an example of a complete Qmail-based multi-domain, web-administerable mailserver including granular and delegatable web administration, fast webmail, and high-performance IMAP and POP3 delivery.

For more details and information than you really want, please see http://www.qmail.org or the companion website http://lifewithqmail.org