When the email sent from Notes Client and the return receipt is marked, the Office365 sent the return receipt some part as Content-Type: multipart/mixed or Content-Type: message/rfc822 which is displayed correctly. The part which has Content-Type: application/ms-tnef; name="winmail.dat" showing as winmail.dat.
At the domino end, it is trying to convert and getting below errors due to incorrect format.
TNEFConvert: ParseFile failed.
TNEFConvert: Encountered errors process message for email@example.com
We need for return receipt winmail.dat need to be removed before delivering to notes client as it is just a confirmation of mail delivery/read.