How to filter "challenges"?

Hi all -

I'd like to classify and discard "challenge" messages sometimes received
in response to my email. An example of these messages is appended below.
Since these messages come from a variety of sources they seem difficult
to classify. Does anyone have a method for detecting these "challenges" or
suggestions on making the recipe below more comprehensive?

:0
* ^Subject: Re:
{
        :0
        * From: SpamLion@
        { STATUS = challenge }

        :0
        * B ?? http://
        * B ?? \<(click|URL|link)\>
        * B ?? \<(anti(-)?spam|Registration|spam guard(ing)?)\>
        * B ?? at [a-Z]+@([0-9]|[a-Z])+\.[a-Z][a-Z][a-Z]
        { STATUS = challenge }
}

To explain the recipe, many challenges seem to involve (not unlike spam) a URL
and directions on loading it. Many of them also contain the email address of
the guarded recipient and a phrase like "antispam" or "spam guard". (The recipe
above would of course be applied after conventional spam-filtering and
greenlisting.)

Thanks!

James


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