There are two postage systems in place now. These provide FREE computational evidence that email is Ham (good) not spam (bad). This makes the filtering process easier, as spammers are not going to spend several seconds sending each email to prove they're mail is worthwhile. Existing versions are Hashcash & mbound. Microsoft is working on a third, similar to mbound.
Spamassassin currently supports hashcash & there is a RFE filed for supporting mbound. Thunderbird (extension) already supports both.
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