See this page for a summarized history of Microsoft's Mac e-mail clients:
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http://blogs.msdn.com/gilg/archive/2004/08/18/216777.aspx>
The missing part from the end is that in response to overwhelming demand for
Exchange support for Mac OS X, Microsoft decided to make Entourage for OS X
the Mac Exchange client by adding Exchange features to an update for
Entourage X. Entourage 2004 has more complete Exchange support, and an
upcoming update will improve that even further.