code: protected function createMessage()
      {
          $config = $this->messageConfig;
          if (!array_key_exists('class', $config)) {
              $config['class'] = $this->messageClass;
          }
          $config['mailer'] = $this;
          return Yii::createObject($config);
      }
docs: Creates a new message instance.
  The newly created instance will be initialized with the configuration specified by [[messageConfig]].
  If the configuration does not specify a 'class', the [[messageClass]] will be used as the class
  of the new message instance.
  @return MessageInterface message instance.