Yahoo, the world's largest email provider, is intending to launch a pay-per-email charity drive that it hopes will curb the rising tide of spam - which currently accounts for over 90 per cent of all email sent.

CentMail revives the idea of paying for each mail sent but with the twist that all proceeds are donated to a charity of the user's choice.

The system, developed by Yahoo Research, uses 1 cent digital stamps that users pre-purchase in blocks of 500 or 1000. 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.

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In a technical white paper, Yahoo Research suggests that CentMail provides a win-win solution all-round. Users get to promote their favourite cause to friends, may potentially attract matching donations from sponsors and, hopefully, don't find their messages ending up in increasingly aggressive spam filters.

Recipients have the assurance of knowing that any CentMails they receive are likely to be from legitimate sources while the charities, obviously, get increased support and visibility. CentMail suggests that as most households already make charitable donations, there would be no extra financial burden on users, and cites the success of Facebook's Causes application that is already used by upwards of 20 million people.

The researchers say, "We do not expect to see complete adoption of CentMail. But if enough senders join, recipients may take notice and begin to whitelist stamped email, allowing them to tune their content-based filters more aggressively, increasing the incentive for senders to stamp email and so on."

Yahoo Research even imagines the CentMail replacing tedious Captcha codes: "The system can serve to rate-limit and validate many types of transactions, from weblog comments to web links to account creation."

There's a CentMail website up at http://centmail.net/, where you can register your interest.