How to send e-mail on an event

Question

I was wondering how to send an E-mail after an event. In this example I'm using stripe payments and I want a mail to be send after a transaction was completed.

Below I have tried to send a mail when a user is subscribed, but I don't receive an e-mail (setTo with an actual email), I don't get any errors either.

my code:

Event::on(Orders::class, Orders::EVENT_AFTER_PROCESS_WEBHOOK, function(WebhookEvent $e) {
                $data = $e->stripeData;
                $order = $e->order;

                if($order) {
                    $user = Craft::$app->getUsers()->getUserByUsernameOrEmail($order->email);
                    
                    if($user) {
                        switch ($data['type']) {
                            case 'checkout.session.completed':
                                $subscription = $order->getSubscription();
                                $oldGroups = $user->getGroups();
                                if($user->isInGroup('newUsers')) {
                                    $newUserGroup = [];
                                    Craft::$app->getUsers()->assignUserToGroups($user->id, $newUserGroup);
                                }
                                if($subscription->planNickName == 'Light') {
                                    $newGroups = [1];
                                }
                                if($subscription->planNickname == 'Standard') {
                                    $newGroups = [2];
                                }
                                if($subscription->planNickName == 'Premium') {
                                    $newGroups = [3];
                                }
                                //send mail
                                $message = (new Message())
                                    ->setSubject('A subscription has been completed!')
                                    ->setHtmlBody('....')
                                    ->setTo('email');
                                Craft::$app->getMailer()->send($message);
                                Craft::$app->getUsers()->assignUserToGroups($user->id, $newGroups);
                                break;
                        }
                    }
                }
            });