Brad: "I’m building code that will send email on behalf of users. It will not receive mail. If I get lots of free users sending millions of mails, how do I pay the per-email fees?"

You mean, like Plaxo? Or sms.ac? No, thanks. Every spammer can make a case their spew is wanted. If you want to build a new service sending millions of mails, I'm fine with the idea that you don't get to fill millions of inboxes unless you've earned the necessary trust -- with an impeccable reputation, or borrowed credentials from your users, or yes, payment of a credentialling fee or bond or per-message postage.

(I also prefer the Dysonesque system of postage refunds: if the recipient doesn't claim the payment with a complaint, you get your postage back. But we'll have to experiment with various payment systems to possibly get there.)

Barring a postage system, your innovation won't be facing evergreen meadows where every message effortlessly finds its delighted recipient. It will face a deteriorating paranoid mail exchange environment where your mere pattern of large-scale messaging or a few ornery user complaints could get you blacklisted with little recourse. At which point you'd have to expend cash-equivalent resources to try to get through, with no guarantees of success.

If the postage rate for an upstanding fellow like yourself works out to less than the effort you'd have to spend in its absence to get the same rate of delivery, then what's the complaint? The postage system would then be less burdensome on your speech and innovation than the alternative. Your objection should not be one of category, but of cost -- and let's see what the actual market prices turn out to be before condemning the "sometimes-senders-pay" mechanism.

(I wanted to table anonymity, but since you bring it up:

Anonymity is already under assault by the current regime of spam filters: the same people who would choose ISPs with postage-protected inboxes are likely to choose ISPs who blacklist anonymous remailers. So I don't see postage as making things any worse. And if you figure on anonymous cash of any sort -- Chaum-esque, or hashcash, or credits for anonymous Turing work, or credits for received mail -- then anonymous senders can pay postage, too. Perhaps net postage balance liquidity will bootstrap a golden age of anonymous commerce! I'm not holding my breath on that one, though.)