Recipient address rejected: User unknown in virtual mailbox table (state 13).

Running a SBS 2008 server with Exchange 2007. Email has been working fine up until just the other day. The problem I'm about to describe currently affects only one user. This user was added to AD about 2 weeks ago. Email for this account has been working both internally and externally. All of a sudden, emails from the outside are rejected with "Recipient address rejected: User unknown in virtual mailbox table (state 13).", and only for this user. None of the previously existing accounts experience this issue. To troubleshoot I deleted the account and mailbox and recreated it. No change. I also created a new account/mailbox. This account experiences the exact same problem. Both of these accounts are able to receive email from senders in the domain and neither have any problems sending.

Any help would certianly be appreciated.

Thanks................