I have an issue where a particular customer is sending emails to one of our users on our Exchange server but they are not receiving them and the server is sending out an NDR. Other users on the same customer domain send emails out and are accepted by the same user .
What can I do to find out why this address is not being accepted? I have added their domain IP's to the Whitelist on exchange but still those emails are rejected.
How can I troubleshoot this issue?