"ZCS comes in five versions. The Open Source Edition,
which is the one I tried, doesn't have all the features of the
others, but it's purely open source.
"Zimbra claims all the editions run on Mac OS X and a variety of
Linux platforms, including Red Hat Enterprise Linux (RHEL), Fedora,
Ubuntu, Debian, Mandriva, SUSE Linux Enterprise Server (SLES),
rPath (a software appliance ISO image), and VMware (a certified
virtual appliance). I installed ZCS 5.0.20 on both openSUSE 11 and
SLES 10 SP2."