Search Mail.app for email messages matching the search query passed.
A few have reported with this not working so I went back and added the option in mdfind to search in a specific folder so that hopefully it would find those results. Report any issues
UPDATE: Based on schoeps comment below, I realized that the search I was performing was queueing off of kMDItemKind == 'Mail Message' and depending on your local language, that may not be the appropriate value. So I changed it to search on kMDItemContentType which should be the same across the board. I have confirmed that it works in English and Italian at least.
UPDATE: I removed the -onlyin argument from mdfind. Removing the specified folder helps in finding the files with alternate language OS's.