User gets NDR – When the the mail has to be moderated by Exchange Transport
This is because of Missing arbitration mailboxes or Corrupted arbitration mailboxes
If you run the below command in Exchange management shell
You should see your arbitration mailboxes , If you see the above result
Then there is no problem with your arbitration mailboxes
If your are not seeing any arbitration mailboxes or you are seeing that your arbitration mailboxes are corrupted.
No wonder why Your Transport moderation does not work as expected and keeps generating NDR
You can recreate them , Even if they go corrupted
How to recreate System Mailbox , FederatedEmail & DiscoverySearchMailbox in Exchange 2010
Arbitration mailbox The arbitration mailbox is used to store the original message that’s awaiting approval. By default, one arbitration mailbox is created for moderated transport during setup. It’s used for all moderated recipients. You can add additional arbitration mailboxes for load balancing purposes. If you’re using multiple arbitration mailboxes, you need to specify which mailbox to use for each moderated recipient.
All the mails which has to be moderated are getting stored in your arbitration mailboxes.
Most common issue on why this happens –
When you delete your default databases abruptly using adsiedit.msc or any other tools
Then you end up missing your arbitration mailboxes
Result of it –Transport moderation does not going to work