docs: def _check_values(self):
          '''
          Make sure all values are of the appropriate
          type and are not missing.
          '''
          if not self.__api_key:
              raise PMMailMissingValueException('Cannot send an e-mail without a Postmark API Key')
          elif not self.__sender:
              raise PMMailMissingValueException('Cannot send an e-mail without a sender (.sender field)')
          elif not self.__to:
              raise PMMailMissingValueException('Cannot send an e-mail without at least one recipient (.to field)')
          elif (self.__template_id or self.__template_model) and not all([self.__template_id, self.__template_model]):
              raise PMMailMissingValueException(
                  'Cannot send a template e-mail without a both template_id and template_model set')
          elif not any([self.__template_id, self.__template_model, self.__subject]):
              raise PMMailMissingValueException('Cannot send an e-mail without a subject')
          elif not self.__html_body and not self.__text_body and not self.__template_id:
              raise PMMailMissingValueException('Cannot send an e-mail without either an HTML or text version of your e-mail body')
          if self.__track_opens and not self.__html_body:
              print('WARNING: .track_opens set to True with no .html_body set. Tracking opens will not work; message will still send.')