The qmail program is a secure, reliable, efficient simple message
transfer agent.  It is meant to be a replacement for the entire
sendmail-binmail system that most UNIX hosts use.

Although qmail holds security and reliability as its top two
priorities, it is also fast.  On a Pentium under BSD/OS, qmail can
easily handle 200000 separate messages per day that are injected
and must then be delivered to local mailboxes!

Security and reliability are qmail's two strengths, however.  The
qmail package ensures a message, once accepted, will never be lost.
An optional new mailbox format, maildir, even lets users safely
read their mail over NFS, while still accepting new mail deliveries.

The following features are supported: host and user masquerading,
full host hiding, virtual domains, null clients, list-owner rewriting,
relay control, double-bounce recording, arbitrary RFC 822 address
lists, cross-host mailing-list loop detection, per-recipient
checkpointing, downed host backoffs, independent message retry
schedules, a drop-in sendmail replacement, and more!

http://pobox.com/~djb/qmail.html
http://www.qmail.org/