The Postfix server is used only for mail-relay; in the office back-end, we use hMailServer. Some hMailServer users have their emails auto-forwarded to their BlackBerry account. Sometimes, they changed their BlackBerry operator without informing IT Dept. As a result, the auto-forward (handled by Postfix) failed, and a notification is sent (by Postfix) back to the user's account, which gets re-forwarded to the (no longer active) BlackBerry account, another failure, and ... well, you get the idea: A mailing loop.
I need to prevent this from happening. For complex reasons, we can't change our current setup of a separate internal mail server and the cloud-based Postfix mail-relay. So, I must either reconfigure Postfix or use some other mail-relay system that can be configured as such.