Note: The rule mention below might catch a majority of the messages, but if POP/IMAP clients are being used, it might not capture all of them. It totally depneds in client behaviour.

Since its not known if the only time that header is used is with read receipts, recomended more testing in connection with checking for what additional impact discarding messages with that header might have.

1- Remove message header Disposition-Notification-To for mail sends from outside my
organization.

2- Remove message header Return-Receipt-To for mail sends from outside my
organization.



Hope it helps.