I have done an upgrade on drupal commerce site from 9.5.11 to 10.2.2
After the upgrade, I have encountered an issue where the email receipts on order complete were sent in an infinite loop until max timeout.
After a couple of hours of debugging, I have found that the core of the problem is not a custom presave hook that I have written, or something like that. It is but a commerce core bug, where if "Email the customer a receipt when an order is placed" checkbox on order type was checked,
Unfortunately, I cannot provide more info, than this, except that when I turn on the debugger the commerce MailHandler gets caught in an infinite loop, which trigger the OrderReceiptSubscriber
Closed: cannot reproduce
3.0
Checkout
Well, since this has never been reported before and I can't reproduce this on my side... I don't really know how I could help... We're definitely sending receipts on almost all of our sites... So this is most probably coming from your setup.
Feel free to reopen if you're able to provide clearly reproducible steps on a clean install.