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.