Mail Delivery Problems DirecPC

Recently, I started to have problems reaching a west coast remote user via our company email. We have never had this problem before, but about three weeks ago, all of his email began sitting in the outbound waiting for delivery que on my exchange server. Then about a day later the mail would be returned host not recognized or unavailable. I have reconfigured and erased his custom recipient setting n exchange, and cleared the cashe from my DNS but I am unable to send him mail. DirecPC has been unable to help me, and it is only his account that I have this problem. I can reach everyone, he can reach me, but I cannot send hime e-mail. Anyone have any ideas??