But doesn't this mean that [in certain circumstances] the 'challenge'
email [...] becomes a form of spam in itself[?]
Yes.
Now if only all the people who run naive C/R systems (and even far too
many of the not-quite-so-naive C/R systems) were to realize that....
So I either 1) ignore ALL challenges, 2) answer them all, or 3) need
some kind of filter to figure out which ones are from legitimate
people as opposed to spam-bots.
Or (4) treat them as you would spam: complain about them, and block
repeat (or otherwise egregious) offenders. That's generally what I do,
when I can't easily identify a message that provoked the challenge.
When I can, I usually tune the complaint a bit. There was even once
when I jumped through the challenge's hoop - special circumstances.
So unless there's a hole in my logic here (there probably is), if C/R
systems are widely used, the will become useless,
Bingo.
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