Short version: Is there a way to have Exchange route all email, including internal AD users that would normally be routed directly, through an external gateway? (SMTP, probably a "Smart Host" in exchange nomenclature.)

Longer version: I'm not an email expert/admin/orevencompetent. Inherited an exchange 2003 server, migrating to web-based SaaS provider. To add to the fun, we're also (forced by deadlines) transitioning domains. What we (my boss) wants is any email sent to the new domain to have a copy sent to both domains. Getting mail sent to the new domain/provider to then be copied/forwarded to our old domain/exchange is easy. But we want mail sent from the old domain to the old domain to get sent to the new domain as well.

However: If we route all outgoing exchange mail through the new provider gateway, w/ the new domain forwarding to the old, we'd get an email loop. The "solution" desired is for an exchange user that sends to another exchange user to still be sent via the external gateway, which would in turn be sent to the new domain, and copied/forwarded back to the old domain. Is it possible?

A bit of a strange request I'm sure. And I expect that what we're attempting to do is DoingItWrong(tm). Any better ideas?