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