Postfix is Wietse
Venema's attempt to provide an alternative to the widely-used
Sendmail program. Sendmail is responsible for most of the e-mail
delivered on the Internet. With an estimated 100 million users, that's billions
of messages daily. A stunning number.
Postfix attempts to be fast, easy to administer, and secure, while at
the same time being sendmail compatible enough to not upset existing users.