I have been looking at the OpenGroupware product, which purports to be a MS Exchange replacement that runs entirely on linux. Features include email, calendaring, tasks, et al.

http://www.opengroupware.org

However, the documentation is somewhat skimpy, making this a difficult product to configure (IMHO). However, they have released a preconfigured Knoppix / OGO server cd.

Now available:

http://www.opengroupware.org/en/download/index.html

Haven't run it yet, but I am pulling it down as we speak. I'll follow up if any comments...

Regards,

~paul