As part of Exchange 2010 planning process, one of the areas you must focus on is the Mailbox server design, with focus on determining CPU requirements. "To that end, Microsoft've published guidance that identifies megacycles / mailbox for each message profile for both active and passive mailboxes. This data was collected using a baseline system (2x4 core Intel Xeon x5470 3.33-GHz processors that yields 3333 megacycles per processor core). Note; you must perform a megacycle adjustment to determine the available megacycles your server design can support.
Recently we made a change with respect to how we normalize available megacycles of server platforms with respect to our baseline system," explains Microsoft.