How to disable x400 and send using smtp

Hi -
I'm trying to get (what I thought was) as very simple new Exchange
installation going. I'm testing using OWA, but whenever I attempt to send a
message, an Event ID 290 is thrown, which indicates that it is trying to use
x400 to send the message. This is quite strange to me, since in the Active
Directory definition of the sending user, there is only an smtp email address
(it did originally contain an x400 address, but I've since removed it). The
smtp service is running on the exchange server.

Can some wise, kind soul instruct me how to get Exchange to send using
smtp instead of attempting to use x400?

Many thanks!