@custom_arg.setter<EOL><INDENT>def custom_arg(self, custom_arg):<DEDENT>
if isinstance(custom_arg, list):<EOL><INDENT>for c in custom_arg:<EOL><INDENT>self.add_custom_arg(c)<EOL><DEDENT><DEDENT>else:<EOL><INDENT>self.add_custom_arg(custom_arg)<EOL><DEDENT>
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