Postfix , formerly known as VMailer, is Wietse Venema's attempt to provide an alternative to the widely-used sendmail. Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Weitse has a long and respected history in the field of UNIX security. Qmail is a freeware sendmail replacement written by Dan Bernstein. Bernstein claims that qmail is a secure, reliable, efficient, simple message transfer agent.
Commercial support is also available.
Smail is another freeware sendmail replacement originally written by Ronald S. and Landon Curt Noll and now taken over by a group of developers. Smail comes distributed with many versions of linux. Zmailer is a multi-process mailer, using two daemon processes to manipulate messages. One of these processes is a router, and makes all decisions about what should happen to a message. The other daemon is a message queue manager, used to schedule delivery of messages.