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. OpenSMTPD is part of the OpenBSD Project. The software is freely usable and re-usable by everyone under an ISC license. This package uses standard alternatives mechanism, you may call /usr/sbin/alternatives --set mta /usr/sbin/sendmail.opensmtpd if you want to switch to OpenSMTPD MTA immediately after install, and /usr/sbin/alternatives --set mta /usr/sbin/sendmail.sendmail to revert back to Sendmail as a default mail daemon.