List of Top Free Open Source & Self Hosted Application for Communication - Email - Mail Transfer Agents

Mail Transfer Agents (MTAs) – SMTP servers.

chasquid – SMTP (email) server with a focus on simplicity, security, and ease of operation. Apache-2.0 Go
Courier MTA – Fast, scalable, enterprise mail/groupware server providing ESMTP, IMAP, POP3, webmail, mailing list, basic web-based calendaring and scheduling services. GPL-3.0 C
DragonFly – A small MTA for home and office use. Works on Linux and FreeBSD. BSD-3-Clause C
EmailRelay – A small and easy to configure SMTP and POP3 server for Windows and Linux. GPL-3.0 C++
Exim – Message transfer agent (MTA) developed at the University of Cambridge. GPL-3.0 C
Haraka – High-performance, pluginable SMTP server written in Javascript. MIT Nodejs
MailCatcher – Ruby gem that deploys a simply SMTP MTA gateway that accepts all mail and displays in web interface. Useful for debugging or development. MIT Ruby
Maildrop – Disposable email SMTP server, also useful for development. MIT Scala
MailHog – Small Golang executable which runs an SMTP MTA gateway that accepts all mail and displays in web interface. Useful for debugging or development. MIT Go
OpenSMTPD – Secure SMTP server implementation from the OpenBSD project. ISC C
Postfix – Fast, easy to administer, and secure Sendmail replacement. IPL-1.0 C
Sendmail – Message transfer agent (MTA). Sendmail C
Slimta – Mail Transfer Library built on Python. MIT Python
Stalwart SMTP – Modern SMTP server designed with a focus on security, speed, and extensive configurability. AGPL-3.0 Rust
vSMTP – Next-gen MTA. Secured, Faster and Greener. GPL-3.0 Rust