Distributed spam email filtering

USENET has a system -- called Spam Cancel -- where a central location (namely, a specific USENET newsgroup) maintains lists of spam posts sent to various newsgroups. Then, special USENET clients can retrieve the list of "spam cancels" from the central location, and suppress the spam posts.

Exactly how this works under USENET is not necessarily important -- but perhaps a similar sort of system can be set up for emailed spam. The basis of the idea is as follows: users who receive spam emails report these emails to a central location; the central location verifies that these emails are, in fact, spam, and then publically posts information that a special mail client or mail server can use to identify that piece of spam, and delete it.

Simple spam email takes the form of one message sent to many different email addresses. Each message therefore contains exactly the same text, and filtering these messages is easy.

More advanced spam mailings contain some unique strings, i.e. a unique URL in each email; however, the bulk of the text will probably remain the same, and will be sufficient for a mail server or client to identify a piece of mail as spam.

Of course, additional criteria can be used by mail servers and clients to identify spam -- for instance, the lack of a "to" address in the headers.

If we are willing to sacrifice human supervision, the entire system can be automated. Flooding of the spam submission system by spammers can be fought, and items can be automatically marked as spam if submitted by enough users.

Of course, compliance from mail clients and/or mail servers will be necessary to make this work. Compliance in mail clients makes more sense to me, because that allows users to choose whether or not to use the spam cancel service. Furthermore, Spam Cancel-enabled clients will (hopefully) allow users to submit pieces of spam to the central anti-spam service, keeping the system going.