Filtering and email pricing as solutions to spam

We analyze the success of filtering as a solution to the spam problem when used alone or concurrently with sender pricing, receiver pricing, or both. We find that filtering alone may exacerbate the spam problem in terms of both the total number of spam message sent and the expected number of spam messages received, because it may cause the spammer to send multiple variants of his message to each customer in an attempt to evade the filter. Sender and receiver prices work as expected. The effectiveness of each instrument improves as the levels of other instruments are increased.