Exchange Serv-Unknowen recipient

I have Exchane server in SBS 4.5,a user have e-mail problem as the folowing:
If he send e-mail to outside than it stopped in the server and i got this notification:MS Exchange IMS..Unknowen recipient,Inbounde mail failer.If anybody send hem e-mail than stopped also in the server and does’t reached hem .the sender got this notification :Unknowen recipient.
any idea about this subject,how can I reconfigure this