Well its looks a no brainer to me.

Actually get your head out of the sand (isps) and do something REAL about spam. Since about 95% of email is spam think how much bandwidth would be saved if you actually DID something.

You could then actually do something about DOS attacks and continual probes by compromised kit and save even more bandwidth.

Not only that it would be green too, think of all the power you will save by not having to add more kit to cope with demand to transmit all that spam and all that DOS/probe traffic.

but then back in the real world.....

I guess just adding more bandwidth and moaning about it is simpler.