Exchange 2003, not receiving emails

We use Exchange 2003, we have one particular email (outside our domain), example: Johndoe@Aol.com . when they send us an Email, we don?t receive it.
However, when you look in the Message Tracking, its how it was delivered. In addition, if you put a read receipt on the message, you?ll get a return on the read receipt, but you won?t get the Reply back from Johndoe@aol.com
Anyone have any ideas ?
Thank you