How to enable Return Receipts on external mail database systems

Hi,

Do you know what are the procedures/things to do in order for me to enable return receipt when sending mail to another external domino mail system. Because I have this problem. I need to confirm if my mail has been opened by a recipient from another company (which also uses Notes as their mail system), but when I email him with return receipt enabled (I even checked the field value ReturnReceipt=“1”), I do not receive a return receipt notification when he opens his email.

Thanks.