Instead, qmail has this VERP thing which is supposed to solve some other problem. I don't see any VERP-like addresses in my bounce mails. I don't see any reliable way of distinguishing bounces from qmail from bounces from random crap either.
So... You qmail users can keep bouncing messages I send to bad addresses on your servers. I sure don't mind.
I don't think it'll be a problem though, I only have 3 of them out of 156 bounces.