Hi!
I was using your plugin for a while and it was ok.
Unfortunately I don't remember what happened exactly, but the plugin stopped to send emails and store completed orders into the plugin's CART ORDERS (I have pile of drafted orders in the trash). The debug started to return:
FAILURE :IPN validation failed.
Though the Paypal receives all the payments correctly and after the transaction is completed and the user is redirected, the "thank you" page receives status Completed in the query string parameters (checked with Chrome's inspector).
I've deleted and reinstalled the plugin, but the problem still persists.
Thank you in advance for your assistance!
Alex
https://wordpress.org/plugins/wordpress-simple-paypal-shopping-cart/