Evaluating result of shell command

Below is an extract of a recipe.  It doesn't work, and I know why but 
don't know how to express my intention.
For the 3rd/4th lines, what I want to say is, "Pipe the message to this 
program, but only if you have execute permission".
        :0 fw
        * $     < ${SpamCheckMaxSize}
        * $?    test -x ${SpamCheckProg}
        | ${SpamCheckProg}

How should I do this?

Many thanks,
Steve
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