Craft CMS 3 - stop sending all emails

craftcms-3-stop-emails.env

# .env file

# 1 to kill, 0 or empty for no effect
KILL_ALL_EMAIL=1

StopEmailModule.php

<?php

use yii\base\Event;
use yii\base\Module;
use yii\mail\BaseMailer;
use yii\mail\MailEvent;

/**
 * In case of emergency, break email. :)
 *
 * - Partial implementation just for example
 * - Assumes you know how to set up a custom module
 *
 * @see https://craftcms.com/docs/3.x/extend/module-guide.html
 */
class MyModule extends Module
{
    public function init()
    {
        // Option to kill all email sending with an
        // environment variable.
        Event::on(
            BaseMailer::class,
            BaseMailer::EVENT_BEFORE_SEND,
            function(MailEvent $event) {
                if (boolval(App::env('KILL_ALL_EMAIL'))) {
                    $event->isValid = false;
                }
            }
        );
    }
}