How to deal with self-signed TLS certificates in Laravel's SMTP driver?

Add this at bottom of your config/mail.php

'stream' => [
   'ssl' => [
       'allow_self_signed' => true,
       'verify_peer' => false,
       'verify_peer_name' => false,
   ],
],

this will solve your problem.

Editor's note: disabling SSL verification has security implications. Without verification of the authenticity of SSL/HTTPS connections, a malicious attacker can impersonate a trusted endpoint (such as GitHub or some other remote Git host), and you'll be vulnerable to a Man-in-the-Middle Attack. Be sure you fully understand the security issues before using this as a solution.

Well in that link you provided the solution is straight-forward.

The correct solution is to fix your SSL config - it's not PHP's fault!

In case you are using Laravel 7.0 you can disable SSL verification in SwiftMailer this way (please note that disabling SSL verification is not recommended!):

config/mail.php

'mailers' => [
    'smtp' => [
        'transport' => 'smtp',
        'host' => env('MAIL_HOST', 'smtp.mailgun.org'),
        'port' => env('MAIL_PORT', 587),
        'encryption' => env('MAIL_ENCRYPTION', 'tls'),
        'username' => env('MAIL_USERNAME'),
        'password' => env('MAIL_PASSWORD'),
        'timeout' => null,
        'stream' => [
            'ssl' => [
                'allow_self_signed' => true,
                'verify_peer' => false,
                'verify_peer_name' => false,
            ],
        ],
    ],
],