Add Premium security features to your question to ensure its privacy or anonymity. Learn more about your ability to control Question Security today.
The email address wasn't found at the destination domain. It might be misspelled or it might not exist any longer. Try retyping the address and resending the message.
If that doesn't work, contact the recipient (by phone or instant messaging, for example) to check that the address is correct. If the problem continues, forward this message to your email admin.
For Email Administrators
For more tips to help fix this issue, see DSN 5.1.1 Errors in Exchange Online and Office 365.