We’ve seen a bunch of folks asking recently if their Exchange 2007 CAS server can talk to their Exchange 2003 mailbox server. The answer is <gasp> it depends on what you mean.
It depends on what technology you are referring to and what you mean by “talk.” Are you referring to WebDAV calls? Then yes. Are you referring to OWA? Then sorta. Are you referring to Exchange Web Services (EWS), then no. There are really two things at play here: proxy and redirection. That is, can an Exchange 2007 CAS server serve as a proxy for calls to the backend mailbox server, or can it successfully redirect calls to the mailbox server where the call can be handled.
In Exchange 2003, HTTP calls were simply forwarded on from the front-end to the back-end. The back-end server would receive the HTTP request and handle it and then forward the response back up through the front-end. In Exchange 2007, however, the front-end CAS handles the HTTP request and speaks RPC to the back-end server.
When a request is received by a CAS server, it knows how to figure out who should handle the request – whether it needs proxied to a different CAS, or can be sent directly to an Exchange 2003 or 2007 back-end. For example, when a request is received for the /exchange virtual directory on a mailbox on an Exchange 2003 server, the HTTP request is forwarded to the appropriate server. So in this way, OWA still works from your 2007 CAS – even though the client will see the OWA 2003 interface. The same is true for activesync clients requesting the \Microsoft-Server-ActiveSync vdir.
If an Exchange Web Services call hits an Exchange 2007 CAS and the CAS is able to determine that the mailbox is on an Exchange 2003 back-end the call will fail since proxying cannot be performed.
Here’s more on the topic: