Exchange Connectors

We are currently migrating between domains at present moving from a NT 4, Win 2k & 2003 Server mix! to a Clean Domain with 2003 DCs and 2000 Server App / File Servers.
As part of this process we are moving from Exchange 5.5 SP4 to Exchange 2003. They are on seperate Exchange Organisations/sites different domains etc.

What we are trying to do is migrate the Exchange users slowly a group at a time. What if any is the recommended method for getting these two Exchange Servers to talk to each other so we can route mail between them? Until we have completed the migration.

Erm Help!

Thanks in advance for any advice at all.

P.S. We have a two-way trust set up and working between the domains.