Hi @Sebring
Please note in Microsoft Q&A forum it is recommended to ask one question per thread unless the questions are related.
Since your first question is related to Exchange online, while the second and third ones are more related to Azure AD, I suggest we mainly focus on the first question in this thread.
For the other questions, please consider creating a new thread and add the Azure AD related tags. It would also help you get better answers.
Thanks for your understanding.
- Will the SMTP enabled for specific mailbox (hence user) will support the above?
Yes. If you enable SMTP Auth for specific mailboxes, it would support the above.
Here is a link on how to enable SMTP Auth for specific mailboxes for your reference: Enable SMTP AUTH for specific mailboxes
In addition, there are some other settings in Azure which may block SMTP Auth, for example, Microsoft Security Defaults.
Please also check this link if you run into trouble with sending messages:
Fix issues with printers, scanners, and LOB applications that send email using Microsoft 365 or Office 365
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