opensmtpd

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 "alternatives --set mta /usr/bin/sendmail.opensmtpd" if you want to switch to OpenSMTPD MTA immediately after install, and "alternatives --set mta /usr/bin/sendmail.sendmail" to revert back to Sendmail as a default mail daemon. The package default installation comes with a minimalistic configuration that uses the "mbox" storage format and protects system directories. If you require a custom or less restrictive service configuration, do not modify the default systemd unit file. Instead, create an override configuration at "/etc/systemd/system/opensmtpd.service".

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