Question : get 550 5.7.1. Unable to relay for specific domains....

If users send to two external domains they get a bounce back right away that says:

550 5.7.1. unable to relay (in reply to RCPT TO command)

There are no problems sending to anyone else it's just these two. They have their own e-mail servers so I don't see why it's considered relaying in this case because it's just going out to the outside world to the final destination.

Answer : get 550 5.7.1. Unable to relay for specific domains....

do you have routing groups setup?
the reason I ask is that the bounce is going through your baracuda device. If the Exchange servers are directly connected, then they should not be routing through the baracuda...