Exchange Adding new users

Hi When I add a new user to my Exchange 2007 server I get the following error when sending to this new user:

550 5.1.1 <blichtig@webermessick.com>: Recipient address rejected: User unknown in virtual mailbox table
Last-attempt-Date: Wed, 13 Apr 2016 21:57:13 -0700

This happens when I send from a outside email account. Everyone else is working fine. It is only when we create a new email account on out Exchange Server. Any help would be much appreciated?