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12 Jun 2005
What's the point of e-postage? All the e-mail systems I know would have no trouble handling their incoming mail if they only got mail that their users wanted, even with a little unwanted mixed in. This tells me that the goal of e-postage is some combination of compensating recipients and deterring or at least rate limiting senders.
If you're going to compensate recipients, you either need a single central post office (or a cartel of post offices that trust each other, which amounts to the same thing), or else you need settlements to get the money from the sender's stamp provider to the recipient's.
We presumably agree that we don't want a central post office to meter the mail as it's injected into the system. I have a pretty good idea of what it would cost to build a transaction infrastructure big enough to count the stamps to get the raw data needed for settlements, which tells me that settlements aren't going to happen, either.
So let's think about rate limiting or deterring senders. Perhaps I'm suffering from a failure of imagination, but I don't see useful rate limiting without some cooperation from the recipients. This is for two reasons: one is that recipients don't want to rate limit the mail they want, and the other is that without some way for the recipients to audit the rate limiting, naughty senders will lie and claim they're rate limiting when they aren't.
There's a minor exception here to the cooperation rule for for ISPs and their customers. If I were an ISP, I would rate limit my customers' outgoing mail since anyone who sends a whole lot of mail and hasn't told me they're running mailing lists is a zombie. Although I can imagine various ways to rate limit by charging by the message, it seems a lot easier and more effective to rate limit by rate limiting, either slowing down senders or just firewalling them until they fix their problem. If you charge, you'll have trouble collecting (credit card companies aren't thrilled about penalties other than their own) and spammees won't find the fact that you made extra money from the spam your users sent them very reassuring.
Receipient rate limiting might take the form of hashcash, although that seems too easily circumvented so long as the bad guys have zombies to do their hashing. With better authentication, recipients might be more able to count mail by sender and tell the overeager senders to back off. With agreements a la Bonded Sender, recipients might demand consideration to accept mail from dubious sources, but then you're back to a cartel or settlements to make the demands stick.
So where does this leave e-postage? Other than as a clumsy way to get people to fix their zombie machines, nowhere I can see.
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