1. Check and confirm that the recipient’s email alias is correctly associated with the mailbox.
2. Verify that the mailbox is still active.
3. Investigate if the message was sent to the correct email alias.
4. Check if the mailbox’s email address policy is properly applied.
5. Ensure if connection filters are being correctly processed.
6. Review recpient’s mailbox and transport settings.
7. Contact the recipient’s domain admin if the issue persists.