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Abstract

A number of anti-spam techniques are instances of the general technique of asking the sender of e-mail to make a small expenditure - of money or another resource - to demonstrate that it is not a spammer. This paper proposes a new one: "charity seals", analogous to Christmas seals on paper mail. A good scheme should have the expenditure be one that most legitimate mail users would not mind, but it should be intolerable for the spammer. Donating to charity is something many legitimate users do anyway, and so the perceived pain to the legitimate users may well be less than for yet proposed scheme. There are a large number of such techniques and in order to compare charity seals we provide a characterization of a large number of them. We then argue that the charity seals model achieves its goal, and avoids some of the implementation difficulties of other models.