My emails are stuck in the queue and never send. What’s wrong?

When Asynchronous sending is enabled (Stores > Configuration > Sales > Sales Emails), Magento doesn’t send instantly, it queues messages and relies on the email sending cron group plus the consumers_runner / message-queue consumers to drain them. If cron isn’t running, or the sales.rule.* and email consumers aren’t alive, mail piles up forever. We restart the consumers, pin them with supervisor or systemd so they survive reboots, and verify cron is actually firing.