I have set up an exchange 2007 server and and migrating to an exchange 2003 server. I moved a few mailboxes over to the new server and the users have reported that they are missing some internal emails. I found the mail messages int eh Queue Viewer under the "Unreachable Domain" queue.
I have tried the steps to re-create the routing group connector detailed here: http://technet.microsoft.com/en-us/library/aa997292(printer).aspx
I also deleted all of the routing group connectors and re-created just one of them.
After that I tried to resubmitt the messages in the queue: http://technet.microsoft.com/en-us/library/aa995987(printer).aspx
I still have messages in the "Unreachable Domain Queue" that just won't go anywhere.