In Exchange 2007 the CAS server provides a connection end point for all the client requests except for MAPI (outlook) and Entourage. This helped in improving the processing handled by the back end mailbox servers in turn improving the performance. In Exchange 2010 CAS design Microsoft has introduced a new service called RPC client access service. This intern moves all the RPC and directory (AD query) client access connection to middle tier CAS. So, now onwards MAPI clients no longer connects a mailbox server to open the mailbox and will use the RPC client access service in CAS server to connect to AD and Exchange mailbox server. For directory information they talks to NSPI (Name Service Provider Interface) end point on CAS server, which replaces the DSProxy component (this change has first introduced in exchange 2007).