Barracuda phone server cannot send email through Exchange 2013

I have a Barracuda phone server which is not sending emails through the local Exchange 2013 server which is on the same subnet as the Barracuda.
I have a FrontendTransport rule set up, but the transport logs suddenly stop right after the Barracuda authenticates successfully... and the emails, of course, never arrive.

I have also attached a screenshot of the protocol settings for the FrontendTransport rule. Any idea what I might be missing?
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highdellAuthor Commented:
I should also mention that the Exchange server is domain-joined and the Barracuda is authenticating using domain credentials.

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