Every message in Gmail has a message ID assigned to it. You can find the Gmail Message ID by logging into Gmail, clicking on an email message and then clicking the down arrow beside the “Reply” arrow and then select show original.
If you know the message ID, you can search for the message in Gmail by using the following search scheme: rfc822msgid:<XXXXXXXXXXXXXXXXXXXXXXX> . You may need to include the domain name during the search (rfc822msgid:<firstname.lastname@example.org>).
I hope this helps. This was posted by techspeeder.