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To perform this, from the app's overview page, go to API Permissions > Add a Permission > Microsoft Graph > Application Permissions. Use the search bar to search for "Mail", Select Mail list and tick "Mail.Send" and . Optionally you can tick also tick "User.Read.All" if you want to see the list of users who can use this Azure app. Add the permissions to grant your Azure app the permission to send emails.
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From the Home screen go to Azure Active Directory > Users. You You can see a list of all the valid users, it should be filled with around a dozen dummy accounts and your account. Copy paste your or any of the "User Principal Name" in that list to your request link and this time the request will be valid. From the same menu, you can get userPrincipalName and ID via GET Request (Optional).
Getting userPrincipalName and ID via GET Request (optional)
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