I too get the same/similar reports when I send to some but not all email addressees, which include:
1) Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send messages outside of your organization. Contact
your email admin for assistance.
2) Generating server: HE1PR07MB1644.eurprd07.prod.outlook.com. Remote Server returned '550 5.1.8 Access denied, bad outbound sender'.