I have a hard time beleiving filtering can't catch a good percentage of spam.
Once I see a pattern, I set a filter --- viola', all the matching spam goes in the junk folder.
I would think the larger ISP's could set filters -- even if they were extremely strict, (say 3 "offenses" with matching patterns and the message is canned).
- To: field containing "unlisted recipients"
- From: field containing unqualified email address
- [Header] "Recieved From" contains improperly formed sender/server address
- Body contains links in where anchor text is a URL, but the underlying HREF is not the same URL or is numeric
- Messages with the word "Viagra", "V I A G R A", "Ciallis", etc.., (although lately it seems stocks "tips" are the major offender).
Message which spoof well known targets, (eBay, PayPal, Chase Bank, etc) should be easy to spot. These companies could use some type of digital signature which could be validated and imposters tested against it.
If I can catch this stuff in a filter with the relatievly small number of messages that come my way, (I pick up email for about 20 addresses), I would think any system tossing mail for hundreds or thousands of addresses should be able to electronicaly detect these types of patterns more easily since they have larger amounts of data to sample.
How about a "commericial email license"? Anyone sending more than (X) messages in (X) hours would need to have a certificate of some sort that is encrypted and has a central registry.
And yes, I think commercial mailers should be charged for bulk mail --- the money can go to a to pay restitution to people who are swindled by email or to pay the operating costs of an email registry.
I don't know the answer, but "it's impossible" does not sound like the right answer.