outlook express/exchange server

I have configured my outlook to connect to my exchange server. everything works fine in outlook. I am able to send and recieve email. When I attempt to use outlook express off of the same configuration, I am only able to recieve. I get a "host not found" attachment on my NDR. I have been through the IMS setup and verified everything. The exchange server is connected to a sonic wall with dmz. This is an initial tryout of this server. Everything was working properly on a legacy email program withoutlook express on the client side. Is the dmz security preventing the smtp from working properly or is there something else I should be looking at?