SwiftMailer with Gmail SMTP suddenly stopped working: Connection could not be established with host smtp.gmail.com [ #0] - swiftmailer

Question

I have several websites that use SwiftMailer with Gmail. I haven't touched any of these sites' code, and suddenly all of them no longer work, resulting in Fatal errors: Connection could not be established with host smtp.gmail.com [ #0]
I've searched all over the internet for what may cause this, tried several solutions, but nothing works. Does anyone know if something either changed on Gmail's end (they no longer allow me to connect?) or what else could have happened?
Some things I tried:
open ports on server router (465 for SSL) - didn't help
disable firewall on server - didn't help
try the IP-address of smtp.gmail.com in stead of the domain name, using ping to find out the IP - didn't help
changing settings in the SwiftMailer program (setting certain verification options to false) - didn't help
I know this isn't a very concrete coding question, but I haven't changed anything to my code so the problem literally cannot be in my code. I'm just hoping someone knows what changed 2-3 days ago, and what I need to do to make SwiftMailer work again.
Note: the Gmail accounts' passwords or account security settings such as 2-step verification and sorts also didn't change. Also, using a different SMTP such as the one from GMX.net, the script still works just fine!
Strange hm?

Answer 1

I had the same issue today. I succesfully tried the workaround mentioned in this web.
Something has been changed in Gmail config. I will try a better solution but in the mean time will use this workaround.
Hope it help.
https://github.com/swiftmailer/swiftmailer/issues/544
You could disable the SSL check by modifying the function
"_establishSocketConnection" in StreamBuffer.php. Add these lines
before stream_socket_client command:
$options['ssl']['verify_peer'] = FALSE;
$options['ssl']['verify_peer_name'] = FALSE;

Answer 2

I would highly recommend you look into the logging plugin.
https://swiftmailer.symfony.com/docs/plugins.html#logger-plugin
It is likely to be already installed so all you need to do is follow the instructions in here and if you run the logger it will give you reasons why the email is failing.

Answer 3

While Toni's solution works it requires fiddling around with vendor source which is not ideal. You can also set the stream options programmatically in your code, where you use the swiftmailer $mailer instance, e.g.
$transport = $mailer->getTransport();
if($transport instanceof \Swift_Transport_EsmtpTransport){
$transport->setStreamOptions([
'ssl' => ['allow_self_signed' => true, 'verify_peer' => false, 'verify_peer_name' => false]
]);
}