Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
mailcow - An open source mailserver suite.
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.
WildDuck - Self hosted modern mail server
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.
Synovel Collabsuite - Synovel Collabsuite is the name of a next generation and open source web server for the Linux operating systems that now features the chat and support for the calendars as well.