Enforcing Bulk Mail Classification
  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. 
Evan P. Greenberg, David R. Cheriton