Saul Hansell in The New York Times has written an article about the a new (?) phase in the spam-wars: instead of trying to identify spam, spam-fighters are developing techniques for trying to isolate the good mail instead. This is something that is called a Mandatory White List, and it’s been around for several years. (Five? Ten?) The problem with this approach is that it requires that all of your email senders to participate, or else you don’t get their mail.
The article also obliquely discusses ways that SMTP might have to be changed, including adding digital signatures. And it rightfully points out that any sender-based approach to solving spam is going to be victim to those spam-viruses that take over a person’s computer and sends email out using that person’s identity.
Frankly, I don’t have a spam problem: I use SpamAssassin. It catches 150+ spam messages for my account a day, and I just see one or two of them.