public function sendVerificationLink(array $credentials, Closure $callback = null)
    {
        $user = $this->getUser($credentials);

        if (is_null($user)) {
            return Contracts\VerificationBroker::INVALID_USER;
        }

        $token = $this->tokens->create($user);

        $this->emailVerificationLink($user, $token, $callback);

        return Contracts\VerificationBroker::VERIFICATION_LINK_SENT;
    }