I integrated with PayPal using the HTML Forms method for accepting payments on my website. When a user selects PayPal as a payment option on checking out, I redirect them to the PayPal website using HTML Forms as described in the documentation. I do receive the payment to my PayPal account, however my on receiving the IPN message to my notify_url when I send the notify-validate command I get the error: 403 forbidden. Please assist. Everything is working as it should in sandbox but I start getting this error 403 forbidden.
Try passing a "User-Agent: XXXXXXXXXXXXXXXX" HTTP header in your notify-validate request, where XXXXXX.. is any string > 5 characters
来源：https://stackoverflow.com/questions/26013941/why-do-i-keep-getting-a-403-forbidden-with-paypal