Mail Server
Open source alternatives in the Mail Server category
Courier IMAP
GPL-2.0
IMAP and POP3 server
Cyrus IMAP
BSD-3-Clause
IMAP server
Dovecot
MIT
Open source IMAP and POP3 email server
Exim
GPL-2.0
Mail transfer agent for Unix systems
Horde
LGPL-2.1
Web application framework and groupware
iRedMail
GPL-3.0
Open source mail server solution
Kolab
GPL-2.0
Open source groupware suite
Mail-in-a-Box
CC0-1.0
Easy-to-deploy mail server in a box
mailcow
GPL-3.0
Dockerized mail server
Maildrop
GPL-2.0
Mail delivery agent
Mailpile
AGPL-3.0
Fast, secure email client
Mailu
MIT
Simple yet full-featured mail server
Modoboa
ISC
Mail hosting made simple
Open-Xchange
GPL-2.0
Open source groupware and collaboration platform
Poste
GPL-3.0
Complete mail server
Postfix
IPL-1.0
Fast, easy to administer, and secure mail server
Sendmail
Sendmail
Mail transfer agent
SOGo
LGPL-2.1
Open source groupware server
Zimbra
GPL-2.0
Collaboration software suite with email and calendar