How to Fix It
The address may be misspelled or may not exist. Try the following:
• Retype the email address then resend the message.
• Clear the recipient nickname cache in Outlook or Outlook Web App by following the steps in this article: Error code 5.1.1 in Exchange Online and Office 365.
• Contact the recipient (by phone or instant messaging, for example) to check that the address is correct.
• The recipient may have set up mail forwarding to an incorrect address. Ask them to check that any forwarding they've set up is working correctly.
If the problem continues, forward this message to your email admin.