 * MimeTreeParser::MessagePartList
  * MimeTreeParser::SignedMessagePart
   * MimeTreeParser::MimeMessagePart
    * MimeTreeParser::TextMessagePart
     * MimeTreeParser::MessagePart
    * MimeTreeParser::EncapsulatedRfc822MessagePart
     * MimeTreeParser::EncryptedMessagePart
      * MimeTreeParser::SignedMessagePart
       * MimeTreeParser::TextMessagePart
        * MimeTreeParser::MessagePart
