Connection could not be established with host with swift mailer in laravel 5.6

Question

I have implemented swiftmailer in laravel 5.6. It was working fine last 2 months. But last 2 days it is not working.

It says:

Connection could not be established with host mail.zinismart.com.bd [php_network_getaddresses: getaddrinfo failed: Temporary failure in name resolution #0]

I can't find out what is my problem.

Here is my code:

// mail driver = sendmail
$transport = new \Swift_SmtpTransport('mail.zinismart.com.bd', '587', 'tls');
$transport->setUsername('test@zinismart.com.bd');
$transport->setPassword('password');

$transport->setStreamOptions(array('' => array('allow_self_signed' => true,"verify_peer_name"=>false, 'verify_peer' => false)));