-- Senders may respond to the challenge by solving some kind of a puzzle -- one that's easy for a human but hard for an automatic spam generator.

-- Alternately, senders can ensure the delivery of their messages by making credit card-based "micropayments." The payments may go to the recipient, the Internet service provider or a charity, or they can be refunded to the sender if the message turns out not to be spam.

"We thought if we could put all that together, we'd have a great long-term solution," Goodman says. "Obviously, it's a very ambitious plan, and I don't think we ever thought it would happen quickly."

Elsewhere at Microsoft, researchers are working on a prototype called MailScope that monitors e-mail routes and alerts users when significant delays are expected. If MailScope sees persistent delays between, say, Microsoft.com and Berkeley.edu, it warns users on those servers that delays are likely, much as a traffic report notifies drivers of congested routes.

In a related Microsoft project called SureMail, when a message is sent, a system posts a tamperproof notification to a table somewhere on the Internet. E-mail recipients periodically query the table and match notifications with messages received. If they find a notification for which there is no message, they know the message has been lost. Microsoft calls these "silent" losses because they so often go undetected. In controlled experiments over two months, using a variety of e-mail systems and carriers, Microsoft found that one in 140 e-mail messages disappeared without a trace. Delays averaged four minutes but lasted as long as 27 hours.