MDaemon
MDaemon Email Server is an email server application with groupware functions for Microsoft Windows, first released by Alt-N Technologies in 1996.

Features 
Mdaemon supports multiple client-side protocols, including IMAP, POP3, SMTP/MSA, webmail, CalDAV, CardDAV, and optionally ActiveSync for mobile clients and Outlook, and its Connector for Outlook add-on.

MDaemon's features include a built-in spam filter with Heuristic and Bayesian analysis, SSL and TLS encryption, client-side and server-side email and attachment encryption, public and shared folder support, mailing list, and support for sharing of groupware data (calendar, contacts, tasks & notes). It is also the basis for MDaemon Tech's Security Gateway for Email Servers.

According to SecuritySpace.com's Mail (MX) Server Survey from May 2020, MDaemon provides approximately 7.6‰ of all known Internet mail servers, the fifth largest installation base from all identified servers.

References

Windows Internet software
Proprietary software
Message transfer agents
Groupware