* [Exim](https://www.exim.org/) - Message transfer agent (MTA) developed at the University of Cambridge. ([Source Code](https://github.com/Exim/exim))`GPL` `C`
* [Exim](https://www.exim.org/) - Message transfer agent (MTA) developed at the University of Cambridge. ([Source Code](http://git.exim.org/exim.git))`GPLv3` `C`
* [Haraka](http://haraka.github.io/) - A high-performance, pluginable SMTP server written in JavaScript. ([Source Code](https://github.com/haraka/Haraka)) `MIT``Javascript`
* [MailCatcher](http://mailcatcher.me/) - Ruby gem that deploys a simply SMTP MTA gateway that accepts all mail and displays in web interface. Useful for debugging or development. ([Source Code](https://github.com/sj26/mailcatcher)) `MIT``Ruby`
* [Maildrop](https://github.com/m242/maildrop) - Disposable email SMTP server, also useful for development. `MIT``Scala`