Thank you for posting question to Microsoft Q&A.
Based on your description, I understand that the sender receives a notification that the email could not be delivered to you. However, you still receive a copy, which includes a non-delivery report. In this case, please try the troubleshooting steps below as I've suggested.
Step 1: Add the sender to the Safe Senders (This step helps the email system fully trust messages from that address, ensuring no emails from the user you've set up are blocked).
- Log in to Outlook on Web
- Go to Settings
- Choose Mail
- Then Junk email
- Add the sender's email address to Safe senders.
Step 2: Check if automatic forwarding is enabled and if the email address being forwarded to is yours. If not, please turn it off.
Step 3 : If you are a company/organizational user, please go to the Exchange Admin Center and check the following features:
- Message Trace in the Exchange Admin Center to see detailed information on how EOP processed the email and find the exact cause of the Non-Delivery Report. This also includes the rules that were applied and the email's spam confidence level (SCL) > Message trace in the new EAC in Exchange Online | Microsoft Learn
- Mail Flow Rules: It's possible that some rules have been set up at the tenant level, affecting all users > Manage mail flow rules in Exchange Online | Microsoft Learn
Once you've completed the troubleshooting steps, please check your private messages so I can assist you in resolving this issue.
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