e-mail accounts could send to this recip

None of your e-mail accounts could send to this recipient. User running Office XP – W2k. I have a user who travels alot. He uses RAS to get his email. For some reason, when he uses the RAS, he will get the above message. The strangest is thing is he will recieve email but on all sent, he gets that error. We are running Exchange 5.5 on a Windows 2k domain.(plan to upgrade soon). If he does his email offline and then plugs into the network, he doesnt have a problem send that email. It is RAS only. I tried to wipe out his .ost and recreate it and checked all of his ras settings. Anyone see this before?