The OpenBSD base system contained a working and secured
sendmail setup for
a long time. It was replaced by OpenSMTPD in OpenBSD 5.3 (2013).
OpenSMTPD was written by OpenBSD Developers from scratch. A simple mail daemon was needed for the base system of the operating system and there were no suitable alternatives. The goals are simplicity, security and reliability with an acceptable license (ISC).
table aliases file:/etc/mail/aliases table secrets file:/etc/mail/secrets listen on lo0 action "local" mbox alias <aliases> action "relay" relay host smtp+tls://firstname.lastname@example.org \ auth <secrets> match for local action "local" match for any action "relay"
OpenSMTPD was audited and has a portable version that is available in various Linux distributions.