Sender Policy Framework

What about the cracked, open-proxy DSL machines that are spam sources today?

Spammers are increasingly using compromised machines on fast connections to send spam.

Broadband providers will have to either force port 25 to go through their servers where they can keep an eye on traffic, or suffer the reputation costs of having their "forged" domain name in the envelope sender line. I know people who block Hotmail, Yahoo, and AOL simply because those are the most commonly forged domains; that hurts their legitimate users and ultimately it hurts them.