Actually I got one today like that. They do usually say which quiz it is, but this one didn't - is it possible there is a wee glitchette somewhere? It said the "Great Quiz!" as the subject line, which is what the 'proper' compliment notes from the link on the quizzes say, and isn't particularly what someone would type themselves if that makes sense.
It is better to open your eyes and say you do not understand, than to close your eyes and say you do not believe.