Side rant:

"GMail lets you write filters against virtually anything"

GMail inexplicably doesn't let you filter against almost anything in the headers, except the few fields they hand-pick. Which is unfortunate because virtually every piece of political junk spam from one major US party has the same thing in its headers, and I can't filter on it. Presumably the other major US party has similar large vendors but I don't happen to get spam from them at this time.