Kontact, a part of the Kolab project, has some very nice Groupware features that were presented on the MK09.
Fortunately most of these are very well usable even without a Kolab server.
Unfortunately these are not so well documented (or the documentation is not very easy to find).
When trying out these features in noticed that the groupware features will only work properly (at least with Kontact from KDE 4.3) if you access your mailbox via “Disconnected IMAP”. If you fail to do so you’ll probably get an “Write access denied” error.
Please note, that Kontact is very unstable sometimes, but again, this depends heavily on the version of Kontact/KDE you use. My experience is based on KDE 4.3 from Debian unstable.
Right, it is called unstable for a reason …