We don't see the cost of transporting email, but it surely exists.
However, as we're just brainstorming here, let's not start editing the
list of topics just yet.
Also, don't assume that "economic model" = "postage". The attention
bond mechanism is a very clever idea with the potential not only to
fight spam but to enable entirely new models of transaction and
exchange by email. If you don't mind a little math, I'd recommend:
http://www.citi.umich.edu/u/rwash/projects/spam/spam-ssrn04.pdf
http://www.citi.umich.edu/u/rwash/projects/spam/spam-slides-stiet-
2003-09-30.ppt
I'm still not convinced that anything like this is deployable, but I'm
fairly well convinced that it would be a really neat thing if it were.
-- Nathaniel