 * MimeTreeParser::MessagePartList
  * MimeTreeParser::SignedMessagePart
   * MimeTreeParser::MimeMessagePart
    * MimeTreeParser::TextMessagePart
     * MimeTreeParser::MessagePart
    * MimeTreeParser::AttachmentMessagePart
     * MimeTreeParser::MessagePart
    * MimeTreeParser::AttachmentMessagePart
     * MimeTreeParser::MessagePart
