* [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`
* [OpenSMTPD](https://opensmtpd.org/) - Secure SMTP server implementation from the OpenBSD project. ([Source Code](https://github.com/OpenSMTPD/OpenSMTPD)) `ISC``C`
* [OpenSMTPD](https://opensmtpd.org/) - Secure SMTP server implementation from the OpenBSD project. ([Source Code](http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.sbin/smtpd/)) `ISC``C`
* [Postfix](http://www.postfix.org/) - Fast, easy to administer, and secure Sendmail replacement. `IPL``C`