Hi @MrJim Johnson ,
Based on your error message “550 5.7.1”, you can refer to this document:
https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange-online/fix-error-code-550-5-7-1-in-exchange-online
You can get in touch with Outlook Sender Support via going to this direct link:
https://support.microsoft.com/en-us/supportrequestform/8ad563e3-288e-2a61-8122-3ba03d6b8d75
In addition, please pay attention to protect your private information, thank you.
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Hello, I'm here to confirm that is there any progress in your question?