Assuming your postfix configuration isn’t doing something to alter bounce delivery, I believe having DISCOURSE_NOTIFICATION_EMAIL in app.yml using the same domain that reaches mail-receiver should be all it needs.
E.g. if your mail-receiver.yml config uses MAIL_DOMAIN: discourse.example.com (and your DNS records result in email to that domain being delivered to mail-receiver), DISCOURSE_NOTIFICATION_EMAIL should look something like whatever@discourse.example.com