framework/mail/BaseMailer.php
protected function saveMessage($message)
    {
        $path = Yii::getAlias($this->fileTransportPath);
        if (!is_dir($path)) {
            mkdir($path, 0777, true);
        }
        if ($this->fileTransportCallback !== null) {
            $file = $path . '/' . call_user_func($this->fileTransportCallback, $this, $message);
        } else {
            $file = $path . '/' . $this->generateMessageFileName();
        }
        file_put_contents($file, $message->toString());

        return true;
    }
protected function saveMessage($message)
    {
        $path = Yii::getAlias($this->fileTransportPath);
        if (!is_dir($path)) {
            mkdir($path, 0777, true);
        }
        if ($this->fileTransportCallback !== null) {
            $file = $path . '/' . call_user_func($this->fileTransportCallback, $this, $message);
        } else {
            $file = $path . '/' . $this->generateMessageFileName();
        }
        file_put_contents($file, $message->toString());

        return true;
    }
Saves the message as a file under [[fileTransportPath]].
@param MessageInterface $message
@return bool whether the message is saved successfully