I have been reading lots of articles recently about how to combat spam. One thing some of the articles have touched on is the problem of authentication. Email doesn't authenticate where the email comes from or is going. It is trivial for spammers or viruses to fake the From address and the return path. It is trivial for them to send their email through open relays or blast it directly to the victim mail servers.
One possible solution is to introduce authentication into the SMTP protocol. This wouldn't protect the From: header in mail messages. But it would protect the return path that is used for bounce messages. It can also be used for access control. This is the difference between knowing the message came from your friend Bob, and knowing that some email@example.com sent the message.
Introducing public-key cryptography into the protocol would not be too hard. SMTP has a mechanism for extensions and adding commands. However, any public-key signature system would depend on distributing the keys and enabling the access control. This requires infrastructure to regulate the sending of email. It requires more centralization in deciding who can send email. It also requires organizations to buy into the system before it helps in limiting the spread of email.
One way to help with the infrastructure problem is to have companies that provide the authentication services. They would run relays that sign messages for its customers. The customers would sign up for accounts, with either monthly or per-email fees to limit the amount of email that could be sent through the system. The relay companies need to be able to authenticate its customers but existing SMTP authentication or SSL cliet certificates are widely supported by email clients.
There would also need to be a mechanism for ISPs to join the system. They would need to get certificates that could be used for signing messages. And even become CAs for creating new certificates for servers and clients.
Authentication would be used to create a group of mail servers that can trust where email is coming from. They can send bounce messages. The postmaster and abuse email works and is answered.
It also helps tie the email system into the legal system. If a spammer breaks into an account and sends a million email that should normally cost $10,000 to send through a commercial relay, that is much more serious offense using some unkown number of open relays. Or forging a cryptographic signature is much more serious than just putting some characters in an email.