docs: def classic_email_groups(self, client_id=None):
          """Gets the list of classic email groups."""
          if client_id is None:
              response = self._get("/transactional/classicEmail/groups")
          else:
              response = self._get(
                  "/transactional/classicEmail/groups?clientID=%s" % client_id)
          return json_to_py(response)