If you look carefully at messages containing spam and e-mail worms, you see some things that arent right. In almost every case, the addressing information that accompanies any such e-mail message is fraudulent. (For more information see "Heading Off Spam".) This is an important characteristic of these messages—and a big part of what allows them to spread so far and be so resistant to defensive measures. A solution is on the way, but it wont come easily.
The solution is SMTP authentication. The idea is that when one mail server receives a message from another, there should be some mechanism for confirming the senders identity. That wouldnt put an absolute end to spam and e-mail worms, but it would put considerable hurdles in their path. It would make effective blacklisting practical, and it would stop the existing, endemic population of worms from spreading.
Why hasnt this happened already? There are a number of proposals in various stages of development, but progress in so fundamental an area takes time. More important, perhaps, is that implementing SMTP authentication would almost certainly require every e-mail server on the Internet to be upgraded and thus would cause considerable disruption and expense—even if the implementation is free of intellectual-property entanglements and direct cost.
Many mail servers out there havent been upgraded in years. It may be impossible to change some, such as those in appliances, and those will need to be replaced. SMTP authentication requires nothing of the end user, though, which is one of the factors that make it so appealing.