Let the cat out of the bag
So in the middle ages they used to hide cats in bags instead of piglets and sell these to idiots? Had these people never heard of spam filtering?
You go down to the bag you oink if it oinks back it is spam and thus worth buying. If it meows it is a cat and you do not buy. How crap were hackers then? It is a straight decision you did not even have to wait for Bayes to be born to come up with that one.