Distributed Quota Enforcement for Spam Control
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, 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.