Exchange – how to deliver internal mail externally

Is it possible to tell Exchange 2007 to deliver all mail to a certain domain originating from the same domain, to external MX'es even though it is set to handle mail for the domain? I.e. not deliver it internally, but forward it to another MX.

Details:

We're in the process of migrating, the official MX is still external, it is only Exchange that thinks it's handling the domain. All users have mail aliases at a second domain (also handled by exchange and with MX records pointing to exchange)

I'd like to keep the MX, but forward all mail for selected users to this secondary domain so that Exchange will handle it, and these select users can access their mail. However, any mail sent to other users in the organization should be sent to the external MX, so it will be accessible at the old server

example:

• domain1 (primary) is handled by an external server (not Exchange)

• domain2 is handled by Exchange, however, Exchange believes it handles domain1 as well

At the external server all mail to someone@domain1 is forwarded to someone@domain2 (Exchange). Here both domains map to the same user.

At Exchange, all mail to anyone@domain1 should not be handled internally, but routed to the external server. The trick is that everything is set up at the Exchange server, but users can be migrated one by one, and start receiving mail at Exchange without other users being affected.