arms race

Spamassassin supposedly does URL scoring now. Yes, there's a shortcoming; but that's obvious, since all spam filters can do against spammers is an arms race. You add a check, they add a workaround. You add another check, they work around that one too. This is a war, one they mustn't win.

I'd think the best way to win that war is to fight back (by wasting their resources for a change, somehow), but of course it has to stay legal. We're the good guys, remember.