Help with accepted domains and emails to nonexistent accounts

We have 6 authoritative domains and 8 internal relay domains in a hybrid exchange environment (one 2013 server on prem and office 365 hosted email server). Out default domain is internal relay (fyi). The issue is that we are seeing emails coming in to non-existent email addresses being bounced back between our server and a barracuda spam appliance. This is filling up the queue. While it is causing a negligible delay, I have been tasked with figuring out what if any impact we would see by changing our internal relay domains to authoritative and if it would prevent the bouncing of emails between the server and the barracuda. We are looking to just dropping emails coming in to nonexistent email accounts.

Thanks for any assistance as I have not had to previously deal with this. Other methods of stopping the emails from bouncing between the server and barracuda and just being dropped are welcomed.