Not having used GroupWise serverside since 5.0, I can't speak to the current functionality.
I do know that the client STILL sucks all holy arse, though. Had to use it for a week (one of our offices is a NetWare/GroupWise shop) and it was painful. I've never been glad to go back to Outlook before or since. The only thing I've ever met that sucks more is the Notes client.
GroupWise always was nicer at the server end. But with a client like that...
Anyway, none of this helps Andrew. His client has Exchange 2003, and is almost certainly unwilling to junk that investment (it ain't cheap).