If I use symfony/mailer in symfony4.4 on the my localhost with mailtrap, it works. No error
But If I try on the server https://my.ionos.fr, it display error like this :
Connection could not be established with host "ssl://null:465": stream_socket_client(): php_network_getaddresses: getaddrinfo failed: Name or service not known at /vendor/symfony/mailer/Transport/Smtp/Stream/SocketStream.php:138)"} 
Can anyone please help me? I have tried in .env but no one is working.
Source: Symfony Questions