We present a plausible path toward adoption of email postage stamps—an oft-cited method for fighting spam— along with a protocol and a prototype implementation. In the standard approach, neither senders nor recipients gain by joining unilaterally, and senders lose money. Our system, called CentMail, begins as a charity fundraising tool: Users donate $0.01 to a charity of their choice for each email they send. The user benefits by helping a cause, promoting it to friends, and potentially attracting matching donations, often at no additional cost beyond what they planned to donate anyway. Charitable organizations benefit and so may appeal to their members to join. The sender’s email client certifies each outgoing message with an unforgeable stamp issued by the CentMail server. The recipient’s email client verifies with CentMail that messages are appropriately stamped, and have not been queried by an unexpectedly large number of other recipients. More generally, the system can serve to rate-limit and validate many types of transactions, broadly construed, from weblog comments to web links to account creation.