I have a mail server that serves multiple domains. I have 2 users that use a specific domain. User A does not experience any issues when sending and receiving mail. User B can send emails fine and receives emails fine from most external users. However when Sender1 sends an email to UserB , the sender gets a "#550 5.1.1 RESOLVER.ADR.RecipNotFound; not found ##" message. However Sender1 can send to UserA just fine. I suspected that the issue was UserB was not setup correctly in exchange. After verification I determined this was not the issue. I then sent UserB emails from a number of external email addresses outside of our domain (Google, Yahoo, MSN) all of which were delivered, as they should have been. I am now out of guesses as to what the issue could possibly be. Any takers?