How to Send From a M365 Shared Mailbox
To send from it you would have to:
Click on New Email at the top of Outlook

Make sure your inbox selected, click on From, click on Other Email Address.
Type in the username for the email. In this case SMS.

It will populate the email address in the From box.
Send a Test email to your email address.

Once you do that it will have your recent From addresses in the drop down box.
Just choose that address any time you want to send from it.

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