mailcow - An open source mailserver suite.
HEY - Email at its best, new from Basecamp.
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.
InstAddr - Disposable email address application.
Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.