Regardless it has been a very long time since I saw one of those
[C/R] schemes in use. I suspect because my anti-spam filter like most
others these days filters out the challenges as spam.
You're lucky, I see them all the time, from Earthlink on down, mostly
blowback due to vast amounts of forgery on my gurus.com domain.
I would be perfectly happy to include C/R in both a taxonomy of spam and
a taxonomy of anti-spam techniques. Most anti-spam techniques don't work,
so you won't get much of a taxonomy if you only list the ones you like.
R's,
John
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