exchange 2002 can’t email out

I have 3 servers in 3 locations. Each runs MS Exchange. Two locations work fine. All emails are under the same domain, but set up under their respective server.
One location cannot email recipients under the same domain that are not set up on its server. Always gives a 550 invalid recipient.
No headers with the email to clue as to routing. Seems like Exchange does not find email locally, so it kicks it back. any ideas?
Thanks.