docs: def available_domains(self):
          """
          Return list of available domains for use in email address.
          """
          if not hasattr(self, '_available_domains'):
              url = 'http://{0}/request/domains/format/json/'.format(
                  self.api_domain)
              req = requests.get(url)
              domains = req.json()
              setattr(self, '_available_domains', domains)
          return self._available_domains