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

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