Microsoft Exchange Server is recommended for medium to large organizations that require a secure, customizable, and robust email solution, especially those already using other Microsoft products. It is ideal for businesses that prefer on-premises solutions with full control over their data and infrastructure, or for those in regulated industries with specific compliance requirements.
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
mailcow - An open source mailserver suite.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Roundcube - Web-based IMAP email client
DistroWatch - 9 hours ago - It does include a proprietary software set optimized for business users, students, researchers and developers." A list of features and further details can be found in the company's release announcement.
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.