Qmail
Qmail (often "qmail") is a mail transfer agent (the email equivalent of a post office) for Unix. Qmail was written by Daniel J. Bernstein because he was fed up with repeated security holes in sendmail. Only two minor bugs have been found in Qmail since version 1.0, and there is an as yet unclaimed $500 prize for the first person to find a security hole in it.Qmail's major competitors are Exim and Postfix. Qmail is distinguished by its highly modular architecture, its adherence to Bernstein's rigorous philosophy of software design, and its security. To date, no security hole has been found in Qmail.
Qmail is free to use and redistribute and the source is publicly available. However, permission is required to distribute modified versions, so it is not free software.
Qmail uses maildirs as opposed to mbox files to store mail and can use the QMTP and QMQP protocols.
See also: DJBDNS.