When using Apple Mail, you should be able to search for a term in From, To, Subject, & Entire Message. However, today I could no longer search Entire Message. It was grayed out & completely unavailable.
I found interesting info on the following pages, with the last being the most helpful:
I closed Mail and tried this, which re-indexes the entire hard drive in Spotlight:
sudo mdutil -E /
But it did nothing. Then I did this, which re-indexes just the Mail folders in Spotlight:
That fixed it.