Actually, a variant on that that might be acceptable� Make e-postage a deposit-based thing. If the recipient has previously white-listed you or marks your particular message as �desired�, then you get your postage back. If not, then your postage is put into the recipients e-postage account to offset the cost of their emails.
Thoughts?
You could have bought the patent on this WKBI on eBay last year: http://www.ebay.com/itm/251279133681 When I was running the ASRG, I set up a wiki where we could keep a taxonomy of anti-spam techniques, so we could save time and just point people at it when they reinvent them. It's still there, contributions of anything we've missed are still welcome: http://wiki.asrg.sp.am/wiki/Attention_bonds R's, John PS: Well Known Bad Idea