document the rt-spam-blocklist filter
in #41016 (closed), it took me a while to figure out that the mails were being dropped by the spam filter, but I couldn't have figured out on my own how to workaround the problem (which was to delete the offending emails) without help from @lavamind.
the fix, it turns out, was to take the header check out of the global configuration and move it into the smtp-specific configuration, but that's besides the point: there's no documentation on our goodie script here, and there should be.