For other systems
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 emails with other systems speaking the SMTP protocol.
Started out of dissatisfaction with other implementations, OpenSMTPD is a fairly complete SMTP implementation.
OpenSMTPD is primarily developed by Gilles Chehade, Eric Faurot, Charles Longeau and Sunil Nimmagadda, with contributions from various OpenBSD hackers.
OpenSMTPD is part of the OpenBSD Project. The software is freely usable and re-usable by everyone under an ISC license.