mailcow - An open source mailserver suite.
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.
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.
Jam.py Postfix Aliases - Web App to manage only Postfix Aliases with a LARGE number of managed Domains (>hundreds).
Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).