Find the file that corresponds to a certain message in Mail.app?
I'm tempted to say the easiest way to do a file contents search in what Finder would consider a System location would be EasyFind - freeware, though it's one app I'd happily pay for. [No affiliation]

You can drag a folder to define the search domain, then search by Phrase in File Contents. You can either QuickLook the result, or right click &amp; open in Mail or Reveal in Finder

I picked a fairly innocuous monthly summary from Backblaze as an example…



One thing to note is that the same mail may appear in more than one location. I really don't know why this is.