问题
In PayPal's IPN simulator, if I enter a https:// URL, I get an error that says "We're sorry, we could not send an IPN." However, on an http:// URL it works fine. How can I resolve this error?
回答1:
This is likely because your SSL certificate is self-signed or otherwise deemed "invalid" by Paypal's IPN service.
Check your website's certificate in a modern browser like Chrome or Firefox by visiting the site over https and then clicking icon next to the https part of the url. If your site's certificate is bad, the https part of the URL will be grey or red instead of green and the clicking the icon will reveal more information about exactly why: for instance if your certificate is self-signed or expired.
来源：https://stackoverflow.com/questions/20015292/why-doesnt-paypals-ipn-simulator-work-with-https-addresses