Last time I looked, Thunderbird still did not work with exchange. Maybe thats changed, but as I have not done anything with Exchange for years I am a bit out of touch on that one.

I was considering open source, and I use open source for my firewall, proxying and filtering services. My custom built filtering solution is better than anything on the market in my opinion. But I have not seen anything open source that has the functionality of exchange or this Mdaemon Messaging server.