OS X Mail: Search within message (including header)
After months of assuming there was no solution to my inquiry, I stumbled on a way to search the entire contents of an individual OS X Mail Message, including headers.

First open the email message you want to search in it's own window. Before initiating your search, select the menu item View &gt; Message &gt; Raw Source or alternately Option-Command-U from the keyboard. This will bring forward a new menu with the entire plain-text content of your message, including extraneous information such as headers.

If you search this Raw Source version of your message using Command-F or Edit &gt; Find &gt; Find… from the menu, the search will include everything in the message, including the headers.


  Note: Although I'm currently using OS X Mail version 6.5, I verified that this technique also works at least back to version 5.3.