What about SPF/Caller ID/MARID/Son of SPF...

Lots of interest from people for this.

Problem is - it doesn't stop spam.

Experiment, late 2004, on an active 50,000 entry greylist.

Use MAIL:SPF perl module, check each entry in greylist
Could only remove 25 entries
But over a thousand had SPF entries indicating they were ok!
All of these entries were spam!

The biggest publishers of SPF records are big ISP's, but....

The most numerous publishers of SPF records are Spammer throwaway domains :)

Spam Permitted From?