Mailu - Mailu is a simple yet full-featured mail server as a set of Docker images.
mailcow - An open source mailserver suite.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
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.
Axigen - Axigen is an integrated email, calendaring & collaboration platform, built on a Linux mail server technology.