Using classic Outlook for Windows in business environments
The screenshot indicates a forwarding failure, not a general mailbox storage problem.
Use this check path:
- Open the non-delivery report and scroll to Diagnostic information for administrators.
- That section usually shows the exact reason the message could not be delivered.
- Common causes include an invalid destination address, a forwarding rule pointing to a bad address, or a delivery rejection.
- Check whether the mailbox has forwarding or rules configured.
- A similar symptom can happen when mail is being forwarded to another address that is no longer valid or is misconfigured.
- In Outlook on the web, check Rules and Forwarding settings and remove any forwarding entry or rule that sends mail to an incorrect external address.
- Verify whether the message actually reached Microsoft 365.
- In the Exchange admin center, go to Mail flow > Message trace.
- Run a trace for one of the failed messages.
- Check the result:
- Failed = the message was rejected; open the details for the exact error.
- Quarantined = the message was stopped by filtering.
- Delivered = the message reached the mailbox and may have been moved by a rule or hidden in the client.
- Check Outlook on the web.
- If mail appears in Outlook on the web but not in the desktop app, the issue is likely the Outlook client or sync/view settings rather than mail delivery.
- If the sender received a bounce message, review that error code.
- NDR or SMTP codes such as 550 5.1.1 or 550 5.7.1 usually identify why delivery failed.
If the failure is specifically because a message is being forwarded from the mailbox to another address, the fix is usually to remove or correct the forwarding rule or forwarding destination.