The Kolab Project provides a powerful groupware server that works together with a variety of clients (Outlook, Kontact, Thunderbird, Horde Webmail ...) and is a complete free software solution.
The groupware server has traditionally been packaged for the OpenPKG distribution which you can install as a "distribution within a distribution" on many Linux distributions. The Kolab2/Gentoo project tries to provide packages that allow you to install the server directly in the Gentoo environment without using OpenPKG.
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