One of the more problematic approaches to spam is a walled garden, an e-mail systems that lets its users exchange mail, but doesn't let mail to or from the rest of the world in or out. Walled gardens are easy to keep spam-free, since the management can set rules and eject people who misbehave.

Why didn't the Internet didn't have any spam for the first decade of its existence? Because it was a walled garden. You could only get in if you were an ARPA or later NSF contractor, there were clear rules, and people could be and were ejected for abuse. Has there ever been a system that allowed cheap communication, didn't have an abuse problem, and wasn't a walled garden? I don't think so. As soon as fax machines became cheap, we had a junk fax problem. As soon as robocallers became cheap, we had a robot junk call problem.

The phone system by its nature is considerably more secure than the Internet, and it's very hard to make a phone call that can't be traced. (You may have to subpoena the info from the phone companies, but that's a detail.)

This suggests that the authentication schemes that people are working on, sich as SenderID (SPF+Caller ID merged) and Domain Keys will be useful if they can provide a level of authentication comparable to what the phone system has. It also suggests that we still need better anti-spam laws, since the junk faxes and robocalls are only kept in check by rather draconian laws that outlaw them completely. For spam, we can only hope.