A newly created recipient may not receive e-mail messages for up to 4 hours
Thanks to Susan Bradley for bringing my attention to this Microsoft KB. It looks like in Essential Business Server 2008 if you create a new recipient and attempt to send them mail, it can take up to four hours for the Exchange Edge Server to synch up to the Messaging Server unless you manually run the Start-EdgeSynchronization cmdlet and force a synch, resulting in a 550 5.1.1 User unknown NDR being generated.
See Microsoft KB 961303 for the details. Looks like they are still updating the KB as of this time.
Steve