An increased load for the CAS
It’s obvious that the relocation of the MAPI endpoint to the RCA layer running on CAS servers has transferred workload from mailbox servers, specifically additional CPU processing. For this reason, Microsoft recommends that Exchange 2010 designs should use a ratio of 3:4 for CAS servers to mailbox cores when you determine server configuration for a site. For example, if the London site has eight mailbox servers, it will need six CAS servers to handle the incoming connections from Outlook clients. This is a very simple calculation and the final number of CAS servers required to handle the load depends on the following factors:
The number of mailboxes hosted on each server and the size of an average mailbox.
The mix of ...