public function GetMailMIME() {
    $result = '';
    switch($this->message_type) {
      case 'inline':
        $result .= $this->HeaderLine('Content-Type', 'multipart/related;');
        $result .= $this->TextLine("\tboundary=\"" . $this->boundary[1] . '"');
        break;
      case 'attach':
      case 'inline_attach':
      case 'alt_attach':
      case 'alt_inline_attach':
        $result .= $this->HeaderLine('Content-Type', 'multipart/mixed;');
        $result .= $this->TextLine("\tboundary=\"" . $this->boundary[1] . '"');
        break;
      case 'alt':
      case 'alt_inline':
        $result .= $this->HeaderLine('Content-Type', 'multipart/alternative;');
        $result .= $this->TextLine("\tboundary=\"" . $this->boundary[1] . '"');
        break;
      default:
        // Catches case 'plain': and case '':
        $result .= $this->HeaderLine('Content-Transfer-Encoding', $this->Encoding);
        $result .= $this->TextLine('Content-Type: '.$this->ContentType.'; charset='.$this->CharSet);
        break;
    }

    if($this->Mailer != 'mail') {
      $result .= $this->LE;
    }

    return $result;
  }