Hi @colinYou might already find this NDR (Non-Delivery Report) documentation that might be helpful for the issue you are facing.
550 5.7.708 NDR exception, which is more likely to affect new customers especially those with a Microsoft 365 trial subscription and using the Microsoft Graph API.
The issue should be resolved once you purchase a paid license. Alternatively, if you're an email admin, consider reaching out to Microsoft Support to request temporary release until you assign Exchange Online licenses.