Thank you for posting this in Microsoft Q&A.
I understand you are trying to use OAuth login with Microsoft Entra to allow users with a Microsoft account to log in to your website. The login works fine if the user logs in with an email that belongs to the same email address as the website, but if you test with a personal Microsoft account, the login fails with the error message "AADSTS500200: User account 'firstname.lastname@example.org' is a personal Microsoft account.
AADSTS500200 is an error code that indicates User account from identity provider does not exist in tenant. This error can happen due to various reasons as explained in this document. I would suggest that you go through this document to fix this issue. https://learn.microsoft.com/en-us/troubleshoot/azure/active-directory/error-code-aadsts50020-user-account-identity-provider-does-not-exist
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