docs: def _convert_postmark_to_native(cls, message):
          '''Converts Postmark message API field names to their corresponding
          :class:`Message` attribute names.
  
          :param message: Postmark message data, with API fields using Postmark
              API names.
          :type message: `dict`
          '''
          d = {}
          for dest, src in cls._fields.items():
              if src in message:
                  d[dest] = message[src]
          return d