OAuth2 'invalid_client' error for personal Outlook.com account using Microsoft Identity Platform

Question

Hello,

I am trying to use OAuth2 to allow a server-side Node.js/Nodemailer application to send emails from my @outlook.com account.

I have followed all documentation to register an application in Microsoft Entra ID, but every authentication attempt fails with the error: Error: invalid_client: The OAuth client was not found.

I am certain this is an account-specific issue because I have confirmed that no sign-in attempts (successful or failed) ever appear in my Microsoft Account activity log (account.live.com/activity). This indicates the server is rejecting the application's identity before it even evaluates my user account.

Troubleshooting Steps I Have Already Taken:

• I have created several new App Registrations from scratch, ensuring "Supported account types" was set to "Accounts in any organizational directory... and personal Microsoft accounts."
• I have generated new Client Secrets and have been careful to use the Value, not the Secret ID.
• I have used the Google OAuth Playground (with custom Microsoft endpoints) to get new Refresh Tokens after every credential change.
• I have confirmed my Nodemailer configuration is correct. Could someone from the Microsoft Identity team please investigate why my correctly configured clientId is not being found by the authorization server? There appear to be no other self-service troubleshooting steps available. Client ID: 735f3b8f-1372-4258-9d7f-579c9129ea16 Directory ID: 64c919c8-f48e-41ba-a530-db998e6b0e32