Nodemailer keeps refusing authentication for invalid login (not using gmail)

I'm wrapping my head around the strangest issue ever right now. I'm using nodemailer and postfix to send email.

The following code is NOT working, giving me the following error : Error: Invalid login: 503 5.5.1 Error: authentication not enabled

  let transporter = nodemailer.createTransport({
    host: 'mydomain.com',
    port: 465,
    secure: false, // true for 465, false for other ports
    auth: {
      user: 'no-reply', // user
      pass: 'a-hundred-percent-the-right-password'  // password
    }
  });

But what is even more strange (and close to scary) is that the same code without any credentials specified is working fine! (But of course the email is going in the spams).

Does any body has an idea of what I could have done wrong ?