Messenger Plus! is built on top of Live Messenger. Although you can encrypt your chatlogs, and perhaps add a layer of encryption on the network with scripts. I wouldn't recommend it for professional use. You are still using Microsoft's public service, which is probably not what you want if you are looking for something as private as Communicator.
Microsoft Lync (formerly Microsoft Office Communicator) is more tailored for internal business use. If that is out of your reach, but you do want a private server, take a look at setting up an internal
XMPP-based server.
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