Wikipedia
OpenSMTPD is a Unix system daemon implementing the Simple Mail Transfer Protocol to deliver messages on a local machine or to relay them to other SMTP servers. It has been released on 17 March 2013 with version number 5.3 after being in development since late 2008.
OpenSMTPD is primarily developed by Gilles Chehade, Eric Faurot and Charles Longeau as part of the OpenBSD project. Its design goals include being secure, reliable, easy to configure, supporting the most common use-cases and with source code that can be distributed under an ISC license. Its portable version, like that of OpenSSH, is developed as a child project which adds the portability code to the OpenBSD version and releases it separately. The portable version was initiated by Charles Longeau and adds supports for multiple operating systems including NetBSD, FreeBSD, DragonFlyBSD and several Linux distributions.