hmm, you really think that they would check that, and you really think that the sender address of that email isn't faked?
you would only also spam the sender address by doing this and produce even more spam.

Most of the spam that I would do this do either is not a spoofed address.  It doesn't mean that it is valid but it not someone else's.  I am hoping that it would act just like a mail server when the user account doesn't exist on the server.

Please look into this feature. People are misled about bouncing email taking them off spammer's lists. Correct, this won't work. 

BUT -- this DOES have a useful feature.

What if someone asks you for your email address, but you dont want to reply to them? You want them to think they didn't get your email address correct? You could bounce back to them and they'd think they didn't get through to you. It's evil, yes, but hey -- it's a nice feature!

Yes this feature should be added. Regardless of who uses it and why. Why not make it a plugin. I've been looking hard into it. 

erm... except that bouncing has to occur at the transport level, not the client level for it to "spoof the idea that the email address doesn't exist"