Contact the recipient's email admin, and do one or both of the following: Give them the error and the name of the email server ...

Contact the recipient's email admin, and do one or both of the following:

  1. Give them the error and the name of the email server that returned the error (found in the NDR that the sender received).
  2. Ask them to add your domain or the sender's email address to their list of allowed senders.

It's likely that only the recipient's email admin can fix this problem. Unfortunately, Office 365 support is unlikely to be able to help fix these kinds of externally reported errors.