code: protected function composeMessage($body)
      {
          $message = $this->mailer->compose();
          Yii::configure($message, $this->message);
          $message->setTextBody($body);
  
          return $message;
      }
docs: Composes a mail message with the given body content.
  @param string $body the body content
  @return \yii\mail\MessageInterface $message