You can delete all your PayPal IPN history by truncating the paypal and paypal_session and paypal_payment_status_history tables using phpMyAdmin.
IMPORTANT ALERT:
USE CAUTION, SINCE YOU ARE DELETING INFORMATION THAT IS USED ELSEWHERE, SUCH AS IN HISTORICAL ORDERS.
Deleting that data is NOT necessary, nor appropriate, when using Express Checkout, since IPNs are still received by your store when using Express Checkout, and that information is still important.
The ONLY reason you should purge that data is if you're resetting your store and deleting ALL order history and all customers.
99% of people should NEVER purge that data. You've been warned.
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