I’d read the help file on Spotlight when I first got my Mac, and knew I could enter search criteria like “filename: invoice” to restrict my searches to filenames, and I’d always noticed that Spotlight searches mail messages too.
But it wasn’t till I read this MacWorld article that I realised that I could also use spolight to do searches like “to:sylvia subject:invoice” to restrict my search to mail messages I’d sent to: sylvia” with the word invoice in the subject.
And in fact I can use the same search in Mac Mail too. Which is how I came to find the MacWorld article (and this one too) in the fist place, because I’d become so frustrated with Macintosh Mail’s “single source” search function. Anyway, I now know that if I leave the search source as “Entire Message” I can indeed enter a search for “to:sylvia subject:invoice” to find the email I’m looking for. Still not as friendly as Thunderbird, but I’m happy that I know how to do it now, and that I don’t even need to do the search in Mail – I can do the search in Spotlight just as easily.