The battle against spam has begun. This report describes the defence mechanisms of today and the near future. The defence mechanisms of today attack the characteristics of spam. By doing this they give the spammers the upper hand. They can continue spamming while the defence tries to filter out the unwanted spam. The battle of tomorrow is promising. The new defence mechanisms are trying to destroy the spammers motivation by increasing the cost of sending email (in CPU-cycles or money) or preventing the spammer from spoofing his adress.
The big obstacle of the future is how the new defence machanisms can by incorporated in a new or improved protocol. A new or improved protocol is absoultely crucial if the battle against spam can be won.
The defence mechanisms described in ths report are: Blacklists, heuristic filters, Bayesian filters, Challenge Response, public signatures, laws, SPF, Caller-ID, Habeas, hashcash, Penny Black, Domain keys and more.