Pretend you're a hacker and you need info from point "A" to point "B" pronto...well this supposedly is your new best friend...I haven't tried it yet but if you just take the 3 mins to read it, it's basically self explanatory. Patience is a virtue :)
basically, if an incoming message doesn't have the right "cookie" in the header, your mail server just bounces it back with an "insufficient postage" error, and you never see it. "postage" is specified in bits of hash collision. computing a correct collision takes considerable time, depending on how many bits you need. checking to see if the collision is valid takes no time. so, the sender must spend computing resources (say 15 seconds per email) to generate a proper collision, or the mail will just bounce back... which limits the sender to sending as many emails as he can generate good "cookies" for in a day, e.g. 5760 emails/day with 15 seconds spent computing per mail. that wouldn't hinder a person, but a spammer wouldn't get very far through his list. as computers get more powerful, you just increase the "postage" to compensate. and friends and mailing lists your's on can be put on a "postage-free" list to save them time (=money).
I find you're a little bundle of info! :) So I'm going to add you to my "friends list" just because I like snooping in on you from time to time....talk to ya whenever! :P ...thanx for the tips here and there, I'm finding them useful.