Bogus mail from SMTP Server.

[Rob]

A number of our users are receiving the following error
message. The message could not be sent because the
server rejected you as a user. Server response:'501
bogus mail from SMTP Server. This occurs when the
original sender of an email is requesting a read
receipt. When a user accepts the read receipt, it goes
to the Outbox to be sent to the original sender. When
the user tries to send/recieve thier email, the message
occurs. Please let me know if you have any suggestions.

Thanks,