PayPal Instant Payment Notification Warning

[jukeboxjunkey]

Hello,

I've just received the following email from PayPal. Any ideas why this is occurring? 

*******************************************************************************************

Hello Me,

Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:

http://www.mysite.com/index.php?option=com_nbill&action=gateway&task=ipn&gateway=paypal

If you do not recognize this URL, you may be using a service provider that is using IPN on your behalf. Please contact your service provider with the above information. If this problem continues, IPNs may be disabled for your account.

Thank you for your prompt attention to this issue.


Thanks,

PayPal

[netshine]

What happens when you visit the URL quoted with a browser?

[jukeboxjunkey]

When I access the URL i get the following message:

A problem occurred whilst attempting to process your payment. The following error was returned: Invalid Transaction - A callback was received, but nBill could not verify that it came from Paypal. This could be due to problems connecting with Paypal to verify the callback (in which case, try switching on the 'use cURL' option on the payment gateway settings page), or because a web crawler (eg. a search engine bot) has found the callback link and is trying to index it (in which case, try adding line to the end of the robots.txt file in the root of your hosting account, containing the text: Disallow: /index.php?option=com_nbill&action=gateway), or (less likely) it could be that someone is trying to pretend that a payment has been made when it hasn't. If you keep getting this error and your genuine Paypal callbacks are failing to be processed, you can turn off callback verification on the payment gateway settings page.

Please contact us if you need assistance.

**END**

I have switched the "use cURL" option, and updated the robot.txt file. I'll test it later today.

I was wondering, could there be an issue with a firewall application or SEF/SEO app?

Thanks 

[netshine]

Please try logging in to your Paypal account and going to History->IPN History. If there is an entry for a failed IPN callback, click on its message ID and check the value of the HTTP response code and the delivery status.