FluentSMTP + Outlook (Azure OAuth) – “Credential Verification Failed” & Reauthentication Issue

Hi community,

I'm using FluentSMTP with Microsoft Outlook (Office 365) via Azure App Registration (OAuth 2.0). Everything was working fine initially, but now I’m running into an issue when trying to reauthorize the connection.

When I click to reconnect, I get this error:

"Credential Verification Failed. Please check your inputs."

✅ What I’ve confirmed:

• Client ID, Tenant ID, and Client Secret are all 100% correct — I've verified and re-entered them multiple times from the Azure portal.

• The Redirect URI is correct and added in Azure:

  https://mydomain.com/wp-admin/admin.php?page=fluent-smtp&provider=outlook

• The FluentSMTP Outlook mailer was working fine before this issue started