See also https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting

As an example, add this to your config.php file

$phpmailer_smtpoptions = array(
    'ssl' => array(
        'verify_peer' => false,
        'verify_peer_name' => false,
        'allow_self_signed' => true
    )
);        

which will avoid certificate checking on SSL connections. Quite useful for development systems with self-signed certificates.