-> I employed a service last month that uses a challenge/response setup,
-> if no response to the challenge, I never see the email on my
-> computer, this has kept over 1300 spams off of my system with quite a
-> few of them being worms.

And I work for a company that has a web shopping basket and sends me and
the customer a copy of the order. When the customer sets up a challenge
thing they will not get responses from us and that includes when they
expedite the order and it bounces our response.

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