code: protected function sendMessage($to, $subject, $message, $additional_headers = '', $additional_parameters = '')
      {
          return $this->getTransporter()
              ->transport($to, $subject, $message, $additional_headers, $additional_parameters);
      }
docs: Messages sender : prepare the whole content and send the e-mail
  
  @param string $to
  @param string $subject
  @param string $message
  @param string $additional_headers
  @param string $additional_parameters
  @return mixed