Lol. Thats great
For anyone else who checks this thread and wants to know without wondering what google search string to use (a lot of results are for adding additional headers using the mail() function), it's the following line in your php.ini file:
;Add X-PHP-Originating-Script: that will include uid of the script followed by the filename
mail.add_x_header = On
Mine is set to "On" so I guess this email is not coming from a script being abused on my server as it does not contain the "X-PHP-Originating-Script" string in the header.
Any other guesses as to where it's coming from and how to stop it?