[ Thanks to darkduck for this link.
]
“Open-Xchange: Open-Xchange comes in several versions, including
a free Community version, Hosting and Server Edition, and Advanced
Server Version. The Advanced Server edition allows you to work with
Microsoft Outlook or MS Exchange. The Hosting version is designed
for hosting services and ISPs to provide email and group services.
Only the Community version is free.“Citadel Groupware Server: Citadel works by dividing elements
into ?rooms?, such as email, RSS, chat, and calendar. Citadel works
with the Linux, BSD, Unix, and Solaris OS, and includes features
such as instant messaging and support for mobile devices. Citadel
is easy to install, and there is a high level of online community
support.“Horde: While Horde is not specifically marketed as an
alternative to Microsoft Exchange, it offers a wide range of
features and modules to add-in. Horde is open source. Because of
the many alternatives, it may be difficult for inexperienced users
to set op, but most administration is performed via a web
interface.”