Microsoft 365 OAuth – 422 Forbidden when sending test email with Shared Mailbox

Hello,

I'm having an issue with the Microsoft 365 (OAuth) connection in FluentSMTP.

The OAuth authorization completes successfully, and the connection is created without errors. However, sending a test email always returns:

{
  "code": 422,
  "message": "Forbidden"
}

The sender is a Shared Mailbox, authenticated using its associated user account.

Could you please advise what might be causing this 422 Forbidden error or if any additional permissions are required for Shared Mailboxes?

Thank you.