protected function _isMboxFile($file, $fileIsString = true)
    {
        if ($fileIsString) {
            $file = @fopen($file, 'r');
            if (!$file) {
                return false;
            }
        } else {
            fseek($file, 0);
        }

        $result = false;

        $line = fgets($file);
        if (strpos($line, 'From ') === 0) {
            $result = true;
        }

        if ($fileIsString) {
            @fclose($file);
        }

        return $result;
    }