In Exchange 2000 and 2003, we had a basic front-end and back-end architecture, where the front-end servers accepted requests from clients and proxied them to the back-end servers for processing. An Exchange 2000/2003 front-end server could proxy RPC over HTTP (now known as Outlook Anywhere), HTTPS (OWA, Entourage etc.), POP, and IMAP clients to the relevant back-end servers. The front-end servers also supported multiple referrals to public folder data on back-end servers.
In Exchange 2000/2003, internal Outlook MAPI clients did not use the front-end server at all; they connected directly to the back-end servers via MAPI over RPC. In fact, because the DSProxy component did not run on the front-end servers, you could not point Outlook MAPI clients to the NetBIOS name or FQDN of a front-end server.
With Exchange 2000/2003, the DSAccess component also accessed the Netlogon service on the domain controller and global catalog servers in Active Directory directly via Remote Procedure Calls (RPCs), and then Outlook clients connected directly to the DCs/GCs. Outlook 2000 and earlier connected to DSProxy.
from:http://www.msexchange.org/articles-tutorials/exchange-server-2007/planning-architecture/uncovering-new-rpc-client-access-service-exchange-2010-part1.html