  Spam costs US corporations upwards of $8.9 billion a year, and comprises as
much as 40% of all email received. Solutions exist to reduce the amount of spam
seen by end users, but cannot withstand sophisticated attacks. Worse yet, many
will occasionally misclassify and silently drop legitimate email. Spammers take
advantage of the near-zero cost of sending email to flood the network, knowing
that success even a tiny fraction of the time means a profit. End users,
however, have proven unwilling to pay money to send email to friends and
family.
  We show that it is feasible to extend the existing mail system to reduce the
amount of unwanted email, without misclassifying email, and without charging
well-behaved users. We require that bulk email senders accurately classify each
email message they send as an advertisement with an area of interest or else be
charged a small negative incentive per message delivered. Recipients are able
to filter out email outside their scope of interest, while senders are able to
focus their sendings to the appropriate audience.
