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