task accept auto mail without recipient

we are facing with a strange problem. After is task is created and send to the involved parties as an email, it asks them to press the accept button (This task is in your list, but the organizer needs your reconfirmation.).  As it is expected when they press the accept button the task is added to their list and a prompt comes up and ask them to send a message back to the sender (organiser). As soon as they press the button to send that confirmation message an error occur that can not send message (email recipient is not accepted by the server) and the mail is stuck in the outbox. Opening up the mail to inspect it the problem is that the “To” field is empty and obviously that is the cause of the problem. As soon as the mail is inserted the rest of the procedure is working as expected.

Please do advise on the issue, it is rather difficult to expect the user to detect the problem, understand why the error occurred and rectify it.

Thank you in advanced