Schemes similar to this — calling back to a sending server to verify that a mail was really sent via that host — have been proposed before in several venues, the most high-profile and public being the ASRG list. Here is a message I sent to that list in April 2003 discussing a few of those schemes:
- J C Lawrence’s ‘forward chained digital signatures’ on Received headers
- William at elan.net’s ‘complex callback verification requirying full message tracking server functionality with dns extensions’
- Russ Nelson’s Q249
- Our own ‘porkhash’
I still like this style of system, I think, but in terms of deployability and simplicity, I’m supporting Sender-Permitted From for now — which similarly forces senders to use registered relays for a given SPF-supporting domain, but using DNS as the protocol and IP addresses as the hard-to-forge identity component.
Another bonus of SPF is that it’s simple, easy to implement, has *running code* out there now, and is being pushed strongly by a pragmatic and sane driving person (in the form of Meng Weng Wong). It’s not always easy in the anti-spam field to find a solution like that ;)
BTW, SPF also, similarly, breaks envelope sender forging. However, I agree, this is one egg that has to be broken to help stop spam (or at least force spammers to use their own domains and IPs.)