Hi, do you know what changed in your site or server that is blocking your emails from getting sent?
You might like to carry out the following troubleshooting steps:
Step 1) Please check the following URL How To Setup PayPal IPN.
https://mbrsolution.com/paypal/how-to-setup-paypal-ipn.php
Step 2) Enable the debug logging option in the simple cart plugin (you can do it from the settings menu). Then do a test transaction so you can see if the IPN message is getting to the plugin. The log file will contain information that indicates that the plugin is getting the IPN message and it is processing it. If the log file is empty then it still isn’t getting the IPN. Some plugin or the server is still blocking it.
Step 3) If after going through all the steps above you are still having issues, try an SMTP plugin like the following one.
https://www.ads-software.com/plugins/easy-wp-smtp/
Let me know if the above helps you.
Thank you.