Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
ClearOS - A very good alternative for Windows Server, for free
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
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.
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...