Market Shares of E-mail Servers

This is actually a question that lies in between a technical question and a discussion, but since I hope to stir up some good and free discussion here, I think it’s better to put this under the discussion umbrella.

Does anyone know what is the current market share of the E-mail servers that corporation are using nowadays, especially the Lotus Domino, MS Exchange 5.5 and Exchange 2000? What are some “distinct” factors that would make a network planner to choose a particular brand or technology?