Sendmail acts as a unified "post office" to which all mail can be
submitted.  Address interpretation is controlled by a production
system, which can parse both domain-based addressing and old-style
"ad hoc" addresses.  The production system is powerful enough to
rewrite addresses in the message header to conform to the standards
of a number of common target networks, including old (NCP/RFC733)
Arpanet, new (TCP/RFC822) Arpanet, UUCP, and Phonenet. Sendmail
also implements an SMTP server, message queueing, and aliasing.

Homepage:
http://www.sendmail.org/
