Have a SBS 2003 with Exchange.
One user hade diffrent exchange related problems so I deleted his AD account and made a new with diffrent username, but same e-mail adresses.
So now he have a correct smtp adress and x400 adress.
He recive external mail.
He recive internal mail that are replyed to (newly sent from him)
But he don't recive mail sent to his smtp adress.