Challenge-response Paradigm in Electronic Mail

We point out the weaknesses of many popular approaches to eliminating E-Mail abuse (spam) and argue in favor of the challenge-response paradigm that, in our opinion, is the only viable way to address the problem at its present stage. We also present an E-Mail server (available under GPL) and a publicly accessible free demo service (see http://sfm.cs.ualberta.ca). Our server exemplifies desirable features of an E-Mail handling system that completely eliminates spam while providing for reliable legitimate human contacts including acceptable ecommerce.