code: def _set_custom_headers(self, value):
          '''
          A special set function to ensure
          we're setting with a dictionary
          '''
          if value is None:
              setattr(self, '_PMMail__custom_headers', {})
          elif isinstance(value, dict):
              setattr(self, '_PMMail__custom_headers', value)
          else:
              raise TypeError('Custom headers must be a dictionary of key-value pairs'f _set_custom_headers(self, value):
          '''
          A special set function to ensure
          we're setting with a dictionary
          '''
          if value is None:
              setattr(self, '_PMMail__custom_headers', {})
          elif isinstance(value, dict):
              setattr(self, '_PMMail__custom_headers', value)
          else:
              raise TypeError('Custom headers must be a dictionary of key-value pairs')
docs: A special set function to ensure
          we're setting with a dictionary