I am running a mixed 5.5/2003 environment. I have some clients connected via a WAN link (3 Mbit). A few of the Outlook clients cannot connect to the Exchange 2003 server. But if I move their mailbox back to the 5.5 server, they can get in fine (after changing the MAPI profile to point to the other server). This occurs with straight RPC connections, as well as RPC over HTTP. These same clients can use OWA (my OWA and RPC over HTTP servers are the same box). Deleting and recreating the profile does not help.