  Unsolicited email (spam) is still a problem for users of the email service.
Even though current email anti-spam solutions filter most spam emails, some
spam emails still are delivered to the inbox of users. A special class of spam
emails advertises websites, e.g., online dating sites or online pharmacies. The
success rate of this kind of advertisement is rather low, however, as sending
an email does only involve minimal costs, even a very low success rate results
in enough revenue such that this kind of advertisement pays off. The anti-spam
approach presented in this paper aims on increasing the costs for websites that
are advertised by spam emails and on lowering the revenues from spam. Costs can
be increased for a website by increasing traffic. Revenues can be decreased by
making the website slow responding, hence some business gets lost. To increase
costs and decreased revenues a decentralized peer-to-peer coordination
mechanism is used to have mail clients to agree on a start date and time for an
anti-spam campaign. During a campaign, all clients that received spam emails
advertings a website send an opt-out request to this website. A huge number of
opt-out requests results in increased traffic to this website and will likely
result in a slower responsibility of the website. The coordination mechanism
presented in this paper is based on a peer-to-peer mechanisms and a so-called
paranoid trust model to avoid manipulation by spammers. An implementation for
the Thunderbird email client exist. The anti-spam approach presented in this
paper breaks the economy of spam, hence makes advertisement by unsolicited
emails unattractive.
