Hi M Smith,
For further troubleshooting, I would like to collect the following detailed information:
1. Were the affected incoming emails sent from external senders? If yes, please confirm whether the same issue occurs if the emails are sent from other mail systems, such as Hotmail.
2. Were the affected incoming emails sent to the on-premise user or Office 365 users?
3. Please provide an entire Non-Delivery Report (NDR).
Thanks,
Claud