As I just posted to MARID, early work on header-based mail server signatures
and signing of entire email (yahoo like) can be found in this paper:
http://www.chaoszone.org/misc/spam.html
This can proof quite usefull in the future as reference of prior work if
we have the same problems as MARID currently does with somebody improperly
claiming patent rights and creating problems with protocol deployment
with restrictive implementation licensing requirements.
(This link came from my old my asrg summary at:
http://www.elan.net/~william/asrg/asrg-may2003-summary.htm
but I totally forgot about it and actually since its not a link to asrg
archive I may have found out about it in some other way)
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William Leibzon, Elan Networks:
mailto: william(_at_)elan(_dot_)net
Anti-Spam Research Worksite:
http://www.elan.net/~william/asrg/