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Abstract

Spam, by overwhelming inboxes, has made email a less reliable medium than it was just a few years ago. Spam filters are undeniably useful but unfortunately can flag non-spam as spam. To restore email's reliability, a recent spam control approach grants quotas of stamps to senders and has the receiver communicate with a well-known quota enforcer to verify that the stamp on the email is fresh and to cancel the stamp to prevent reuse. The literature has several proposals based on this general idea but no complete system design and implementation that: scales to today's email load (which requires the enforcer to be distributed over many hosts and to tolerate faults in them), imposes minimal trust assumptions, resists attack, and upholds today's email privacy. This paper describes the design, implementation, analysis, and experimental evaluation of DQE, a spam control system that meets these challenges. DQE's enforcer occupies a point in the design spectrum notable for simplicity: mutually untrusting nodes implement a storage abstraction but avoid neighbor maintenance, replica maintenance, and heavyweight cryptography.