* [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`
* [MailHog](https://github.com/mailhog/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](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`
* [Dada Mail](http://dadamailproject.com/) - A web-based list management system that can be used for announcement lists and/or discussion lists. (Also has a "Pro" version which allows for more/larger mailing lists and branding removal.) ([Source Code](https://github.com/justingit/dada-mail)) `GPLv2``Perl`
* [Mailman](https://www.gnu.org/software/mailman/) - The Gnu mailing list server. `GPL``Python`
* [Sympa](https://www.sympa.org/) - A mailing list manager. `GPLv2``Perl`