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Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.
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http://blogs.technet.com/b/infratalks/archive/2012/09/07/transition-from-small-business-server-to-standard-windows-server.aspx
For $5/mo/user it's going to take a long time to justify the price of exchange and windows server os and the hardware to run it.