"ZCS, therefore, is more than just open source. Licensed
versions offer additional functionality above and beyond the open
source version are also available. These may appeal to customers
with particular messaging needs. Plus, ZCS runs primarily on
Linux-based platforms, although it is also available for Mac OS X.
There is not a Windows version, but the server can certainly
interoperate with clients on Windows platforms. An open extension
framework is also available that enables two-way interoperability
with leading collaboration platforms, including Microsoft Exchange
Server, Lotus Domino Messaging Server and Meeting Maker."