code: protected function _presendInit()
      {
          // Fournir si possible un Message-Id: conforme au RFC1036,
          // sinon SpamAssassin denoncera un MSGID_FROM_MTA_HEADER
          $sender_mailer = $this->getOption('sender_mailer');
          if (Helper::isEmail($sender_mailer)) {
              preg_match('/(@\S+)/', $sender_mailer, $domain);
              $this->setRegistry('Message-Id', '<'.time().'_'.rand().'_'.md5($this->getMessage()->get('text')).$domain[1].'>', 'headers');
          } else {
              $this->addError(
                  sprintf('!! - Error in "sender" address (%s) - the message will probably be considered as a spam.', $sender_mailer)
              );
          }
      }
docs: Initialization before sending messages
  
  @return void