On the NDR errors, the message means that it could not deliver a notification that a message was undeliverable.
This is most likely because the system account' mailbox that you have configured to receive system messages is full.
Check the mailbox of UNITY_SERVER or whatever you have configured to receive notifications.
You can open this mailbox and delete those messages.
The only parameter that controls the target for NDR messages is the registry key.
So you can create new DL change that registry key and restart Unity for a change to take effect.