Rule for Outlook to only allow mail from our domain.

Just switched to hosted exchange with SherWeb.  So far so good.  However previously with our in house spam product etc, we could created a policy/rule to only allow emails from and to users on the domain.  This can't be done with SherWeb, as they provide Baraccuda Networks spam/virus software as part of their hosted solution for exchange.  You can manage your spam individually as the service catches what it "thinks" is spam in each users mailbox, there they can delete, view, whitelist or send to the "in-box".  Just can't do anything from a "global" settings standpoint.

Is there a way to do this in outlook or should this be approached via a policy in AD, for instance? Any solutions that would accomplish this?