Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
sSMTP - sSMTP is a simple MTA to deliver mail from a computer to a mail server.
mailcow - An open source mailserver suite.
OpenSMTPD - OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with...
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.