What I would do, presuming that these are probably HTML-formatted emails anyway, is to put an arbitrary per-message hash-string into <--- an HTML comment --> within the message text.   If the e-mail is returned to you (by GMail or by any other mail server along the route), you can use the hash-string to verify that it (probably...) is a copy that was returned, and you can look up the information that you need (such as order number) in your own database. (!!)   You can also mark that particular message as “returned” since it is entirely possible that you will get more than one undeliverable-notice.   You can mark the e-mail address as “doubtful,” using the e-mail address that you stored, not the one listed in the body of the message you received.

Also consider digitally signing every message (via S/MIME) so that you can vouch for its authenticity when processing it a second time.   These signatures are generally invisible even when the message is viewed on a web-mail screen.   (I personally think that every computer-generated e-mail should be signed, especially if, as in this case, you intend to react to response messages.   Never assume that the message you got, purporting to be an e-mail rejection message, is, in fact, an e-mail rejection message.   Never assume anything.)