Since this error is happening on a server that's outside Office 365, you'll need to check a few things to fix the problem. ...

Since this error is happening on a server that's outside Office 365, you'll need to check a few things to fix the problem.

Check the NDR for information about where the problem is happening. For example, look for a domain name like contoso.com. The recipient's email server could be causing the problem, or it could be due to a third-party service that's filtering the messages.
Check the NDR for information about how to fix the problem. The NDR should tell you what went wrong and how to fix it. For example, if the message was blocked due to a potential virus, try sending the message again without attachments.

If you can't fix the problem, contact the responsible party's admin. This could be the recipient's email admin, your smart host service admin, or someone similar. Provide them with the error code and error message from the NDR to help them troubleshoot the issue. Unfortunately, Office 365 support is unlikely to be able to help with these kinds of errors.