The original email system

was intended to be used amongst trusted computers. If we want a trusted network, all we need to do is make a trusted server network by hand. By enforcing email server registration, one can be sure that no company would allow spam traffic through it's servers. The server registration can be done the same way as the dns registration, by requiring certain accountability data from all organisations in exchange for their mx record. Mail that comes from a chain that has a server with a missing reverse mx record is almost always spam. (people send and receive files through their service provider or company mail server, they don't directly send from their own computer) This tackes zombie machines.

The other problems are hacked mail servers, which is more trouble than worth for spammers and hacked or fake user accounts that can be filtered, because the sender and the reply address should match and it's not easy to maintain serveral thousand fake email accounts so spammers tend to change the reply address.

The third form of defense could be a central user reported spam database, so if enough users report the same message as spam, a central database could mark the provider or mail account as a spam source. This list can be checked against before accepting a new mail. This database can be abused so care must be taken to avoid false positives and in most cases the first two checks should be enough.

The last solution would be to attack those who use spam for their business, so instead of finding the spammers one can always find some form of contact address otherwise the spammers could not get their money.

The four solutions above would leave only non money oriented spammers operating with hacked servers or accounts in business. For most of them, this wouldn't worth the trouble.