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.
These details are provided for information only. No information here is legal advice and should not be used as such.