public function header_delimiter()
    {
        // use the configured delimiter for headers
        if (!empty($this->prop['mail_header_delimiter'])) {
            $delim = $this->prop['mail_header_delimiter'];
            if ($delim == "\n" || $delim == "\r\n") {
                return $delim;
            }
            else {
                rcube::raise_error(array(
                    'code' => 500, 'type' => 'php',
                    'file' => __FILE__, 'line' => __LINE__,
                    'message' => "Invalid mail_header_delimiter setting"
                ), true, false);
            }
        }

        $php_os = strtolower(substr(PHP_OS, 0, 3));

        if ($php_os == 'win')
            return "\r\n";

        if ($php_os == 'mac')
            return "\r\n";

        return "\n";
    }