I have setup a Barracuda spam firewall to deliver messages directly to one of our Exchange 2007 servers, but I'm running into a problem. If the mailbox that the Barracuda needs to deliver to is hidden from the address list, the mail bounces back with "Unable to Relay". Any ideas? As soon as I unhide the mailbox, delivery works. I'm still scratching my head over whether this is a Barracuda or Exchange issue... Any help is greatly appreciated, thanks!

Turned out the Transport Service on one of the servers didn't pick up the new accepted domains I had added. Restarted the service and all is well.