To fix the problem, contact the recipient's email admin and give them the error and the name of the external email server ...

To fix the problem, contact the recipient's email admin and give them the error and the name of the external email server that reported the error found in the NDR. Ask them to work with their domain registrar to make sure their domain is properly set up and their DNS records are properly replicated across the Internet.

Only the recipient's email admin can fix this. Unfortunately, Office 365 support won't be able to help with these kinds of externally reported errors.