From "Jay Cousins" via NANOG...

Here's my idea for stopping, oh say, 90% of the world's spam. (At least
in the US)

Sorry, we need the Feds to involve themselves a bit here.

By federal law, all servers in the USA must have reverse DNS active
and properly configured. Give 90-120 days for compliance. Then
blackhole any US server... and any other server worldwide, that is not in
compliance. Block them at the top router level. Set up a web site, ala
the Blackhole List, where once the server is in compliance and
compliance is verified, they are unblocked.

Since tracing the spammer's address becomes an easy thing to do, the
market place can go to work in a level playing field. Those who do not
wish to have their servers properly configured can do business
elsewhere.

Since the protocol is already in place, and the policy was originally
stated to be that any server sending e-mail must have RDNS enabled
and properly configured, moving policy to law and then enforcing the
law would be fairly straightforward.

Jay