I'm hoping to get some clarification on the IPN url. We currently use the notify_url parameter to specify the IPN url in a form submission. Is there a requirement that this url be served over https://? I understand that we need to post back verification to PayPal over https:// but I'm unsure about the post from PayPal -> IPN url.
If https:// is required for an IPN url, I'm curious to know about testing in the sandbox. Some of our dev environments don't have certificates and I'm wondering if I can supply an IPN url that is not served over https:// for testing.