protected function _auth()
    {
        if ($this->_helo == false) {
            throw new SMTPException(__('Must call EHLO (or HELO) before calling AUTH'));
        } elseif ($this->_auth !== false) {
            throw new SMTPException(__('Can not call AUTH again.'));
        }

        $this->_send('AUTH LOGIN');
        $this->_expect(334);
        $this->_send(base64_encode($this->_user));
        $this->_expect(334);
        $this->_send(base64_encode($this->_pass));
        $this->_expect(235);
        $this->_auth = true;
    }