framework/log/EmailTarget.php
protected function composeMessage($body)
    {
        $message = $this->mailer->compose();
        Yii::configure($message, $this->message);
        $message->setTextBody($body);

        return $message;
    }
protected function composeMessage($body)
    {
        $message = $this->mailer->compose();
        Yii::configure($message, $this->message);
        $message->setTextBody($body);

        return $message;
    }
Composes a mail message with the given body content.
@param string $body the body content
@return \yii\mail\MessageInterface $message