E-mail

Strange problem
One of the employs is trying to send a e-mail to a customer. The customer doesn?t receive the e-mail.
The customer can send to us. Not even I can send to the customer.
So what I did I went and tried from gmail and webmail. And the customer does receive the e-mail. i can send to a gmail account and it does receive my email.
I did look on exchange system manager.
The message get to our outgoing server and stops.
We using exchange 2003