I use spamnet available from www.cloudmark.com and have VERY few spams make it thru.. out of abotu 200+ spams/day I think I actually see about 10 and it has never blocked a legit emai (including mailing lists) as far as I can tell it uses md5 sums of the reported spams to detect spams.. so basically after enough people classify the email as spam they will automatically flag it.
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