https://marco.org/2009/12/03/keep-this-checked-my-entire-message-search
Keep this checked.
My “Entire Message” search in Mail.app hasn’t worked for a long time, always just inexplicably returning zero results for any search. Today it was finally causing enough of an inconvenience that I searched for a fix.
I learned that for “Entire Message” searches, Mail just uses Spotlight on the message files. So if you leave this box unchecked* in Spotlight’s preferences, “Entire Message” searches simply won’t work, and neither Mail nor Spotlight felt it necessary to tell me this. (You also need to ensure that ~/Library/Mail isn’t excluded from settings in that Privacy tab.)
I was impatient, so after fixing that setting, I told Spotlight to manually import the messages immediately:
mdimport ~/Library/Mail
As soon as that completed, “Entire Message” searches started working.
* I had unchecked it because mail messages always cluttered up the results when I was simply trying to launch an app or find a document.