Modeling Incentives for Email Blocking Strategies

We consider the current arrangements whereby many users send their email via ISP “smarthosts” and model this system as an economic network. Some of these users will have insecurely configured machines, which may be hijacked for the sending of spam. This can lead to other sites blocking all email from the smarthost, which will then affect all of the ISP’s users. We derive formulae that express when blocking is an appropriate action and show that the number of legitimate users at the ISP must be taken into account as well as the number of spam sources. We relate this formal model to real world behaviour and attitudes and generate some strategies that maximise utility for different sizes of ISP. We present some quantitative email data from a large ISP that indicates the likely range in which numerical solutions will lie. We also demonstrate that some of the present approaches to email blocking are irrational and counterproductive.