src/Mail/mime.php
function txtHeaders($xtra_headers = null, $overwrite = false, $skip_content = false)
    {
        $headers = $this->headers($xtra_headers, $overwrite, $skip_content);

        // Place Received: headers at the beginning of the message
        // Spam detectors often flag messages with it after the Subject: as spam
        if (isset($headers['Received'])) {
            $received = $headers['Received'];
            unset($headers['Received']);
            $headers = array('Received' => $received) + $headers;
        }

        $ret = '';
        $eol = $this->_build_params['eol'];

        foreach ($headers as $key => $val) {
            if (is_array($val)) {
                foreach ($val as $value) {
                    $ret .= "$key: $value" . $eol;
                }
            } else {
                $ret .= "$key: $val" . $eol;
            }
        }

        return $ret;
    }
function txtHeaders($xtra_headers = null, $overwrite = false, $skip_content = false)
    {
        $headers = $this->headers($xtra_headers, $overwrite, $skip_content);

        // Place Received: headers at the beginning of the message
        // Spam detectors often flag messages with it after the Subject: as spam
        if (isset($headers['Received'])) {
            $received = $headers['Received'];
            unset($headers['Received']);
            $headers = array('Received' => $received) + $headers;
        }

        $ret = '';
        $eol = $this->_build_params['eol'];

        foreach ($headers as $key => $val) {
            if (is_array($val)) {
                foreach ($val as $value) {
                    $ret .= "$key: $value" . $eol;
                }
            } else {
                $ret .= "$key: $val" . $eol;
            }
        }

        return $ret;
    }
Get the text version of the headers
(usefull if you want to use the PHP mail() function)

@param array $xtra_headers Assoc array with any extra headers (optional)
(Don't set Content-Type for multipart messages here!)
@param bool  $overwrite    Overwrite the existing headers with new.
@param bool  $skip_content Don't return content headers: Content-Type,
Content-Disposition and Content-Transfer-Encoding

@return string             Plain text headers
@access public