docs: def custom_arg(self, custom_arg):
          """Add custom args to the email
  
          :param value: A list of CustomArg objects or a dict of custom arg
                        key/values
          :type value: CustomArg, list(CustomArg), dict
          """
          if isinstance(custom_arg, list):
              for c in custom_arg:
                  self.add_custom_arg(c)
          else:
              self.add_custom_arg(custom_arg)