Exchange 2007 Server - Sender unable to send one recipient in the domain

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http://serverfault.com – 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 o (HowTos)