Non-delivery Reports (NDRs) are bounce back messages sent when journaling is in place. When a journaled message cannot be delivered to the intended recipient, an NDR is created and is sent back to the user listed as the NDR recipient.
On older versions of Exchange, this was not a requirement, however, on O365 you must have a contact listed. It is best practice to set the recipient to a shared mailbox that multiple people can monitor. NDRs are the first sign that something is wrong with the journaling system.
Messages sent to the user set as the NDR contact will NOT be journaled. This is to prevent loops, for example: If an NDR sent to the contact can also not be journaled, you now have two journal reports, and in some cases, could continue to happen indefinitely.
Warning pop-up dialog for O365 when configuring an NDR recipient:
Sample NDR message: