Bounces should go to the "envelope from address", which is different than the from address in the actual email. The way I've set this up before had the envelop

Bounces should go to the "envelope from address", which is different than the from address in the actual email. The way I've set this up before had the envelope from address be "bounce-$user_id@companydomain.com" so that we could identify bad addresses, but any replies from humans go to someone directly. You can also use the Reply-To header to achieve the same result.