Here is a list of steps to troubleshoot this issue:

1. Check that the domain is functioning properly, that the email address is valid, and that the receiving mailbox has not been disabled.


2. Check the email’s SMTP headers to make sure it’s being routed correctly and is not being blocked due to SPAM/malware.


3. Verify whether or not the user’s Outlook mailbox is full or over quota.


4. Attempt to send an email to the specific user from a different email address or system.


5. If all of the above steps fail, try to use an email recovery program to recover the emails.