Postfix attempts to be fast, easy to administer, and secure, while at the same
time being sendmail compatible enough to not upset existing users. Thus, the
outside has a sendmail-ish flavor, but the inside is completely different.

Some features:

Connection cache for SMTP, DSN status notifications, IP version 6, Plug-in
support for multiple SASL implementations (Cyrus, Dovecot), TLS encryption and
authentication, Configurable status notification message text, Access control
per client/sender/recipient/etc, Content filter (built-in, external before
queue, external after queue), Berkeley DB database, LDAP database, MySQL
database, PostgreSQL database, Maildir and mailbox format, Virtual domains,
VERP envelope return addresses and others.

WWW: http://www.postfix.org/