I think it’s a combination of measures that make it more effective, not just one, like security that works in layers you must be able to beat all layers to make it thru, it is a cat and mouse game but most spam is not sophisticated. We can deal with the sophisticated ones later and still have a first few layers of defense. WoT has done pretty good so far, but leaves legit new users out in the cold.

It doesn’t make sense to tackle at the relay level because most relays don’t know what’s happening on other relays, only clients know.