You're right, but my hope is that forcing spammers to switch domains will increase their costs to the point that spamming is no longer financially viable. If t

You're right, but my hope is that forcing spammers to switch domains will increase their costs to the point that spamming is no longer financially viable. If the cost of domains is already such a significant expenditure that they need to look for sub-one dollar registrations, then requiring, say, a $10 bond on all domains with an MX record might erase their profit margins completely. (There is a question of what constitutes "good behaviour" and whether that can be gamed by having spam domains reporting each other as sending legitimate email, but if these ratings are public then people can choose which ratings to trust. Domain age would probably be a good heuristic there too.)