Here's a fascinating new paper  (pdf) from the Journal of Economic Perspectives on the economics of spam, by Justin Rao and David Reiley:
The negative externalities imposed by spam include wasted time for consumers: both wading through irrelevant advertisements in one’s inbox and missing an important message that went to the junk mail folder. They also include the costs important message that went to the junk mail folder. They also include the costs of server hardware, which requires more than five times as much capacity as would be required in the absence of spam, as well as the costs of spam prevention services provided by firms to reduce the burden on users. . .
Taken together, the total costs of spam worldwide today appear to be approximately $20 billion, in round numbers.
The authors review literature on the revenue of spammers, and find that it's likely to be around $300m a year. In other words, spam destroys around $19.7bn dollars of value every year. The authors compare this to car theft, which imposes societal costs of around $10bn and brings revenues to the thieves of around $1bn; and to driving a car, which imposes societal costs of around five cents a mile and brings in average revenue of around 60 cents a mile.
Clearly all these numbers are extremely rough estimates, but if they are even in the same ballpark as the truth then it is clear that spam ought to be a public policy priority to a far greater extent than it actually is. Imagine if the number of car thefts doubled overnight; would people really question whether that calls for governmental involvement?
The problem the authors identify is one of negative externalities, similar to the issues raised by carbon emissions. So one would expect the solution to be analogous to the solution economists often call for to deal with externalities, which is a Pigovian tax - charging someone who harms society an amount equivalent to the damage they do. Unfortunately, as the authors show, such a measure is pretty much impossible when it comes to email spam. The spammers would obviously not co-operate, and every alternative involves trying to graft on some form of payment mechanism to email, which is a protocol incredibly unsuited technologically to any such addition.
Instead, the best thing to do is probably to hit the spammers where it hurts: their revenues.
One fruitful avenue is to put legal pressure on domestic banks that process payments from foreign banks known to act on behalf of spam merchants. This could put downward pressure on conversion rates and with them, proifts. Another proposal comes from our colleague Randall Lewis, who imagines “spamming the spammers” by identifying spam emails and placing fake orders on spam-advertised stores. This step would increase the merchants’ costs dramatically, as they would find it much more difficult to fullfil orders, and their banks may raise their fees if they submit many invalid payment authorization requests. Of course, an unintended consequence is that from time to time, a legitimate merchant will be inundated with bogus product orders.
Commenting on the paper, Digitopoly's Joshua Gans points out  that, under US law, that may not be entirely legal:
A few years back I contacted Yahoo and Google with an idea to counter spammers. What if for each spam email that they picked up, they responded — perhaps entering details into phishing forms? This would overwhelm spammers and they would not be able to find ‘legitimate’ responses from the gullible few. That would really alter their returns. Unfortunately, it was explained to me that such a measure would constitute an attack by a US corporation and, apparently, that is against US law.
Spam may be here to stay, then. The real solutions are technological, and don't involve fixing email so much as abandoning it altogether; the time for being able to accept free, unsolicited email from anyone seems to be coming to an end. Those who are trying to build its replacement  will be happy indeed to hear that.
The full paper is a surprisingly good read; if you're looking for something to flick through on an e-reader over the weekend, why not give it a go?