How do Microsoft Outlook clients communicate with Exchange 2007?

A. Outlook clients running MAPI RPC over TCP (or MAPI RPC over HTTP) communicate directly with the mailbox server. Outlook clients talk to a Client Access server to fetch client profile settings, free/busy data, and the offline address book. All other client protocols are via the Client Access server (e.g., IMAP, POP3, ActiveSync, Outlook Web Access--OWA). It's therefore possible to not have a Client Access server in a location if clients are MAPI only; however, the absence of the availability service (which is where free/busy comes from in Exchange 2007), the offline address book, and so on, would probably cause access problems.