The key to getting bounces sent to the right account is the Email Domain of the Mail Account that you've configured for Bounce Processing. The emails that are sent out use that domain for the "envelope" from address (which is not the one you see as sender or recipient, but the one that's hidden in the smtp conversation between machines).

As more detailed explanation is here: https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/inbound/