Self-Hosted Software Communication - Email - Mail Transfer Agents

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1. 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`

2. Courier MTA

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`

3. Exim

Message transfer agent (MTA) developed at the University of Cambridge. ([Source Code](https://git.exim.org/exim.git)) `GPL-3.0` `C`

4. Haraka

High-performance, pluginable SMTP server written in Javascript. ([Source Code](https://github.com/haraka/Haraka)) `MIT` `Javascript`

5. MailCatcher

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`

6. Maildrop

Disposable email SMTP server, also useful for development. `MIT` `Scala`

7. 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`

8. OpenSMTPD

Secure SMTP server implementation from the OpenBSD project. ([Source Code](https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.sbin/smtpd/)) `ISC` `C`

9. Postfix

Fast, easy to administer, and secure Sendmail replacement. `IPL-1.0` `C`

10. Qmail

Secure Sendmail replacement. ([Source Code](https://sources.debian.net/src/netqmail/1.06-5/)) `CC0-1.0` `C`

11. Sendmail

Message transfer agent (MTA). `Sendmail` `C`

12. Slimta

Mail Transfer Library built on Python. ([Source Code](https://github.com/slimta/python-slimta)) `MIT` `Python`