Solution: NDR Error "550 5.4.1 Recipient address rejected: Access denied" for one user - Exchange Office 365 Hybrid

How to fix non-delivery report error "Remote Server returned '550 5.4.1 Recipient address rejected: Access denied" for one user - Exchange Office 365 Hybrid

Problem:

Emails sent to one user via an Exchange hybrid server to Office 365 bounce back with a Non-Delivery Report (NDR) error message "Remote Server returned 550 5.4.1 Recipient address rejected: Access denied". 

In this case, emails are being sent to an on-premises Exchange 2016 server set up in a hybrid with Office 365, and the messages bounce back from Office 365.

Troubleshooting:

• Check that other users are able to send mail through the Exchange hybrid server. They are, only one user is affected.
  
• Check the users smtp email addresses are correct on the Exchange hybrid server

Solution:

The user was missing the @DOMAIN.onmicrosoft.com smtp email address, and the remote routing address was not set correctly on the Exchange hybrid server.

Notes:

Emails from the Exchange hybrid server are delivered to Office 365 using a send connector. If the email smtp addresses and remote routing address are not set correctly, the messages cant be delivered.