protected function getAuthenticatorsForAgent()
    {
        if (!$mode = strtolower($this->auth_mode)) {
            return $this->authenticators;
        }

        foreach ($this->authenticators as $authenticator) {
            if (strtolower($authenticator->getAuthKeyword()) == $mode) {
                return [$authenticator];
            }
        }

        throw new Swift_TransportException('Auth mode '.$mode.' is invalid');
    }