Along these same lines, I can get a contents/content match to match an email address in an email on the Desktop but I can't get Source URL/Address to recognize author email addresses, email addresses, or recipient email addresses. Nor can I get anything to match with author email addresses, email addresses, or recipient email addresses. Should I be able to?
I'm reindexing to see if that makes any difference.
Related question: Can Hazel monitor the Apple Mail inbox for new mail and move it to another location? Moving mail from the library folders found at ~/Library/Mail/V6 gives a less descriptive file name than the results of files dragged from the inbox. I know you've said elsewhere that Hazel isn't really for handling email so I know this is a stretch. Thanks!
I would highly recommend against having Hazel monitor and process Mail's files. It can impact Mail's operation and possibly result in corruption of your mailboxes.
Your best bet is to have Mail copy those messages somewhere else where Hazel can monitor it.