Patent claim on challenge-response filtering

You guys have probably seen this before, but I searched for "patent" on the
archives on sourceforge.net and didn't find anything, so here goes:

  http://www.siliconvalley.com/mld/siliconvalley/5565843.htm

Incidentally I found this linked from http://spf.pobox.com/ (itself an
interesting project, to give a DNS policy framework for validating sender
addresses)

Regards,

Brian.


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