//This topic belongs in networking, not security.
What it basically does is respond to the remote MTA with a 4xx, meaning "error, try again", forcing the remote server to set aside resources to try again. If that's creative use of error codes or abuse depends on your point of view. Since there isn't any real error, in my book that amounts to abuse. Anyone using this, and those freaks using that thingie where you have to register with them proving yourself a "legitimate sender" (I know there's a term for that can't remember it tho), will never make it out of my blacklists.