Kolab

Kolab Groupware is a Free Software groupware solution for Email communications, Events & Appointments, Contacts and more. It supports mixed client environments because of an open storage format, and the use of well-established, standard protocols such as IMAP and SMTP.

For the full Kolab experience, you need a Kolab server, and one or more Kolab clients. What makes Kolab a unique groupware solution is its focus on security, scalability, data ownership (aka privacy) and the use of proven components and protocols.