The computer at the source would have to solve the puzzle — which would take about 3 seconds — and send the e-mail back. The filter would then let the e-mail through.
The added time might add 0.01 cents to the cost of sending each e-mail. For someone sending 100 e-mails a day, that adds just 1 cent a day — not much of a burden, Gates says. But, "If you're sending a billion pieces of e-mail, that's actually a serious amount of money."
The added cost would force bulk e-mailers to more carefully select e-mail targets, cutting the random e-mails now landing in people's in-boxes. Gates says Microsoft is pioneering this approach.
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