Let me point folks at a paper written by a colleague of mine, Bala Krishnamurthy, on a better charging scheme. It uses contingent liability -- that is, you only pay afterwards if someone thinks you sent spam. See http://www.research.att.com/~bala/papers/shred-ext.ps (or .pdf)
--Steve Bellovin, http://www.research.att.com/~smb
