Contact the recipient's email admin, and ask them to add your domain name or the sender's email address to their allowed ...

Contact the recipient's email admin, and ask them to add your domain name or the sender's email address to their allowed senders list.

While the sender might be able to alter the message content to fix this, 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.