OpenSMTPD is a FREE implementation of the server-side SMTP protocol
as defined by RFC 5321, with some additional standard extensions.
It allows ordinary machines to exchange e-mails with other systems
speaking the SMTP protocol.

Started out of dissatisfaction with other implementations, OpenSMTPD
nowadays is a fairly complete SMTP implementation.

OpenSMTPD is primarily developed by Gilles Chehade, Eric Faurot and
Charles Longeau; with contributions from various OpenBSD hackers.