How to: Communication - Email - Mail Transfer Agents
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[Mail Transfer Agents](https://en.wikipedia.org/wiki/Message_transfer_agent) (MTAs) - [SMTP](https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol) servers.

- [chasquid](https://blitiri.com.ar/p/chasquid/) - SMTP (email) server with a focus on simplicity, security, and ease of operation. ([Source Code](https://blitiri.com.ar/git/r/chasquid/)) `Apache-2.0` `Go`
- [Courier MTA](https://www.courier-mta.org/) - Fast, scalable, enterprise mail/groupware server providing ESMTP, IMAP, POP3, webmail, mailing list, basic web-based calendaring and scheduling services. ([Source Code](https://www.courier-mta.org/repo.html)) `GPL-3.0` `C/deb`
- [DragonFly](https://github.com/corecode/dma) - A small MTA for home and office use. Works on Linux and FreeBSD. `BSD-3-Clause` `C`
- [EmailRelay](https://emailrelay.sourceforge.net/) - A small and easy to configure SMTP and POP3 server for Windows and Linux. ([Source Code](https://sourceforge.net/p/emailrelay/code/HEAD/tree/)) `GPL-3.0` `C++`
- [Exim](https://www.exim.org/) - Message transfer agent (MTA) developed at the University of Cambridge. ([Source Code](https://git.exim.org/exim.git)) `GPL-3.0` `C/deb`
- [Haraka](https://haraka.github.io/) - Fast, highly extensible, and event driven SMTP server. ([Source Code](https://github.com/haraka/Haraka)) `MIT` `Nodejs`
- [OpenSMTPD](https://opensmtpd.org/) - Secure SMTP server implementation from the OpenBSD project. ([Source Code](https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.sbin/smtpd/)) `ISC` `C/deb`
- [OpenTrashmail](https://github.com/HaschekSolutions/opentrashmail) - Complete trashmail solution that exposes an SMTP server and has a web interface to manage received emails. Works with multiple and wildcard domains and is fully file based (no database needed). Includes RSS feeds and JSON API. `Apache-2.0` `Python/PHP/Docker`
- [Postfix](http://www.postfix.org/) - Fast, easy to administer, and secure Sendmail replacement. `IPL-1.0` `C/deb`
- [Sendmail](https://www.proofpoint.com/us/products/email-protection/open-source-email-solution) - Message transfer agent (MTA). `Sendmail` `C/deb`