Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
ClearOS - A very good alternative for Windows Server, for free
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
NethServer - NethServer is an operating system for Linux enthusiasts, designed for small offices and medium...
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.
FreeIPA - FreeIPA is an integrated security information management solution combining Linux (Fedora), 389...