PayPal IPN模拟器无法正常工作

I am trying to send an Instant Payment Notification (IPN) to my server using Instant Payment Notification (IPN) simulator on Paypal Developer site.

I have been searching all over the internet and I found a site which allows to test the ipn http://samples.geekality.net/ipn/ when I use this url http://samples.geekality.net/ipn/listener.php in IPN simulator it shows me the data on this sample site but when I use my own site's url which is something like this http://example.com/appointments/payment/notify it just doesn't work I am sending an mail function using to my email something like this:

mail('example@example.com', 'notify action performed', http_build_query($_REQUEST));

when I access this url directly it works and send me an email but doesn't work with ipn simulator.

FYI: Return/Cancel url works fine.

Q: Am I doing something wrong? And How can I solve it?

Please let me know if this is a possible duplicate.

Thank you.