A. In some cases it's true, you can run Exchange 2010 on the standard edition of Server 2008. But not always. Because Exchange 2010 is 64-bit only, the reason to use the Enterprise edition of Server 2008 has decreased. Even the standard edition of Server 2008 x64 supports 32GB of RAM (the enterprise edition supports 2TB). You would therefore, only need the Enterprise edition of Windows if you have Exchange servers needing more than 32GB of memory.
There's another scenario where you'd have to use the Enterprise edition of Server 2008, however. If you're using Database Availability Groups (DAGs) for mailbox high availability, the Windows failover cluster feature is used behind the scenes, and this feature is only available in the enterprise edition. If you think you'll use DAG, make sure you install your mailbox servers on Server 2008 Enterprise or Server 2008 R2 Enterprise.
Note that you can still use the standard edition of Server 2008 for the CAS, HUB, Edge, and UM roles, as long as they require 32GB or less of memory.
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