[Exim] [offtopic] anti-spam that actually works

I have an idea for an anti-spam system that would actually work. Most
anti-spam is based on either looking at the text for clues, or for the
message source. Neither is reliable.

There is one thing about spam that the spammer doesn't hide. Spam is
sent to lots of destinations.

My proposal is to build a site. It would set itself up to attract
spam, but subscribing to lots of lists, etc. By keeping a hash (md5?)
of messages, it would identify spam by the large number of identical
hashes.

How does the client benefit? By redirecting his mail through this
anti-spam engine, the spam is removed.

I suppose it would be possible for the spammer to counter this by
making his messages unique. But I think this would require
modification of his spam-sending software that is probably beyond the
expertise of most spam senders.

What do you think?