We have one forest with 4 domain trees, we'll call them Domain1, Domain2, Domain3 and Domain4. Our Barracuda is setup to manage 4 total email domains, one for each AD domain. We have users that exist in Domain1 that used to exist in either Domain 2 or Domain3. They still need to be able to receive email at username@domain2 even though their primary email is username@domain1 and they only exist in Domain1. We are unable to make the Barracuda work with this setup. If we setup Domain2 email domain to point to Domain2's DC on the Barracuda, and then an email for a user in Domain1 comes into their Domain2 email address, it's unsuccessful in finding the user to verify. I hope this is making some sense. The only solution I can think of, thus far, is to remove the email addresses relating to Domain2 and Domain3 from the users in Domain1 and then create mail enabled contacts in the respective domains.