[Exim] custom error response

I have setup a local spam filter that rejects by domain and user and
that's working great now thanks to help I received from this list,
thanks! Now I would like to have it add a message saying that the
mail was rejected due to spam from the site instead of or in addition
to the generic "550 rejected: administrative prohibition".

I'm not clear on how to do this, please help :)


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