Here's a possible Challenge-Response:

this is a computer-generated response

Greetings....

Your address is unknown to this program.
Please take a moment to verify that it
is being used by a real human being
that wants to communicate with me, and
not some spammer's computer.

Just hit Reply, paste
 
    <unique password here>
 
anywhere on the Subject line, and send it off.

[ this is a one-time thing ]

Thanks for your patience,

Alan Connor.

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If this is not returned within 72 hours, the mail that triggered it will be deleted, unseen, by elrav1, the program that sent it to you.

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    elrav1 -- http://tinyurl.com/l55a

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