That looks like debugging output from PHPMailer. Check your Postfix logs, it seems to be a configuration problem. The actual mailer log may provide further information for you.
Of course this doesn't appear to be a WordPress issue since WordPress is trying to handover the mail to Postfix and it's Postfix that's failing. I only mention that because if this thread descends into a how-to-fix-Postfix one, it'd be better played out in a Postfix specific forum.