Why do I keep getting a 403 forbidden with PayPal?

Question

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.

Answer 1

Try passing a "User-Agent: XXXXXXXXXXXXXXXX" HTTP header in your notify-validate request, where XXXXXX.. is any string > 5 characters

edit: newer documentation on this point