If there's one lesson from the history of spamming, it's that spammers are among the fastest adopters of anti-spam technologies. If PoW meant that spammers could only send 1/100th the email, but the email was 1000× more likely to actually show up in an inbox, spammers would jump on it in a heartbeat.
The value to spammers isn't in email delivered, it's the actual user engagement at the end of the process. A million messages delivered to the inbox is worth far more than a billion messages delivered to the spam folder.
A million messages delivered to the right inboxes are valuable.
The quantity-above-quality model in the spam industry probably lead to low-quality or vague list criteria. Broad demographic groups and domains. A lot of third party data of questionable legitimacy. If you've been running the list a long time, some "email address foo@bar.com clicked on campaigns A and B" data. You might be able to say on a large scale "List A is likely to convert better than B", but it still comes down to spray-and-pray at the individual address level.
If you went to a spammer with a few "sucker lists" of 100k emails each, and told him "you can only send 1k per day", will he have the data to triage his list to get a decent return on it anymore?
This would also snowball over time. Without being able to do high-volume campaigns initially, they'd have a harder time building up the knowledge they can use to manage their lists, and the quality would decline over time.
Yeah, but if the marginal cost of the next message is higher than the marginal expected increase in revenue then they won’t. It doesn’t matter optimizing inbox delivery rates if per-delivery you lose money.
Keep increasing the PoW cost and think about where that line of argument leads you. "One message delivered to the inbox is worth far more than a billion messages delivered to the spam folder." One single delivered message is going to be profitable for the spammer? Clearly not. This proves the existence of some break-even point at a lower cost. Mail providers just need to set the cost above this point.
Given that spammers mostly use others' computers to blast their email out, I suspect that the price point that would truly throttle spam would be higher than the price legitimate users of email would be willing to pay. I'm not sufficiently knowledgeable in spam and black market economics to throw any actual numbers around.
