private function resolveClient($client)
	{
		$client = ! empty($client)
			? $client
			: new Client();

		$client->setBaseUrl(Properties::baseUrl())
			->setUserAgent('Double Opt-in php-api/' . self::VERSION);

		$httpClientConfig = $this->config->getHttpClientConfig();
		if (array_key_exists('verify', $httpClientConfig)
			&& $httpClientConfig['verify'] === false)
		{
			$client->setSslVerification(false, false);
		}

		return $client;
	}