How to Fix It
The address may be misspelled or may not exist. Try one or more of the following:
Send the message again following these steps: In Outlook, open this non-delivery report (NDR) and choose Send Again from the Report ribbon. In Outlook on the web, select this NDR, then select the link "To send this message again, click here." Then delete and retype the entire recipient address. If prompted with an Auto-Complete List suggestion don't select it. After typing the complete address, click Send.
Contact the recipient (by phone, for example) to check that the address exists and is correct.