Connection could not be established with host smtp.mailgun.org :stream_socket_client(): unable to connect to tcp://smtp.mailgun.org:587 (Connection refused) code example

Example: Connection could not be established with host smtp.googlemail.com

In Laravel project directory, edit config/mail.php and add the following:
'stream' => [
        'ssl' => [
            'allow_self_signed' => true,
            'verify_peer' => false,
            'verify_peer_name' => false,
        ],
    ]
    
It worked for me.

Fyi, my SMTP settings are:
MAIL_DRIVER=smtp
MAIL_HOST=smtp.gmail.com
MAIL_PORT=465
MAIL_USERNAME=[Full Gmail Address]
MAIL_PASSWORD=[Google App Password obtained after two step verification on Google Account Page]
MAIL_ENCRYPTION=ssl