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.
• The recipient may have set up email forwarding to an incorrect address. Ask them to check that any forwarding they've set up is working correctly.
• Clear the recipient Auto-Complete List in Outlook or Outlook on the web by following the steps in this article: Fix email delivery issues for error code 5.1.10 in Office 365, and then send the message again. Retype the entire recipient address before selecting Send.
If the problem continues, forward this message to your email admin. If you're an email admin, refer to the More Info for Email Admins section below.