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The problem of spam is a classic "tragedy of the commons" (G. Hardin, 1968). We propose the Zmail protocol as a way to preserve email as a "free" common resource for most users, while imposing enough cost on bulk mailers so that market forces control the volume of spam. A key idea behind Zmail is that the most important resource consumed by email is not the transmission process but the end user's attention. Zmail requires the sender of an email to pay a small amount (called an "e-penny") which is paid directly to the receiver of the email. Zmail is thus a "zero-sum" email protocol. Users who receive as much email as they send, on average, neither pay nor profit from email, once they have set up initial balances with their ESPs (email service providers) to buffer the fluctuations. Spammers incur costs that moderate their behavior. Importantly, Zmail requires no definition of what is and is not spam, so spammers' efforts to evade such definitions become irrelevant. We describe methods within Zmail for supporting "free" goods such as volunteer mailing lists, and for limiting exploitation by email viruses and zombies. Zmail is not a micro-payment scheme among end-users. It is an accounting relationship among "compliant ESPs", which reconcile payments to and from their users. Zmail can be implemented on top of the existing SMTP email protocol. We describe an incremental deployment strategy that can start with as few as two compliant ESPs and provides positive feedback for growth as Zmail spreads over the Internet.