It worked on my old setup. But I see on newer stuff it doesn't work now, as you stated. I will research that.
Are most of these emails you have problems with also used on websites, forums, facebook (publicly viewable), and the like? Is there some way that multiple harvesters keep hitting them?
You could almost ban anything with an image in the body with a body_check.
ISPC3 on Debian! It's great!