In Exchange 2007, Microsoft introduced “five server roles that performed distinct functions within Exchange organization. One role in particular - Client Access server role - introduced a variety of new Web services, including Availability service, Autodiscover service and Calendar Concierge services. Exchange 2010 has same server roles. However, there’re some significant architectural changes and some shift in responsibilities. Perhaps the most significant change in Exchange 2010, two new services on CAS called RPC CA and Address Book services establish RPC endpoint for MAPI, NSPI and RFR client access. This new functionality replaces RPC endpoints in Information Store. RPC endpoint in Information Store hasn’t been removed in Exchange 2010, but it has been modified to only accept requests from CAS servers. RPC endpoint for public folder database access remains on Mailbox server, however, Outlook clients now communicate directly with RPC CAS on Mailbox Server for public folder database access, and not with Information Store[…],” writes Microsoft.
Full Article: Exploring Exchange 2010 RPC Client Access service