I'm afraid this hasn't worked as I've just tried to close a couple of these InfoPath form requests. It is still bouncing back saying it is 'Undeliverable' and that the email address cannot be found. This is despite the email address being added to the Junk Notification Rule as well as the subject header container 'undeliverable'. See screenshot below which are the default rules provided in SDP.
The InfoPath requests that I'm trying to close are not new requests, but existing ones that were created a couple of months ago when we were testing the forms submit function.