It’s not a traditional bounce receipt – it simply comes from ‘BOUNCE’ – ‘[email protected]’ and won’t process through Google’s servers to the recipient. The reply-to is [email protected].
There is nothing in the headers, and just a cryptic ‘An error occurred. Your message was not sent.’ statement placed above the original message.
I can forward you the headers if you like.
1.6.22 with the same configuration does not result in the ‘BOUNCE’ message from gmail’s servers, it’s only 1.6.23d.