We are having a strange issue in magento 1.9. Our customers are receiving order confirmation emails of the orders which are already completed.
What could be the cause of this issue?
Since sometime around Magento 1.9.x (I can't recall exactly) new order emails are queued in the database instead of being sent immediately. A scheduled task (cron job) is then responsible for sending the emails out from this queue.
I would hazard a guess that your cron job has not been running and has now been enabled or fixed, and now all the historical new order emails stored in the
core_email_queue table are being sent out.