Paypal Issue

Hi there
I need some help here please.

On my website I am using Paypal as my account merchant.
Everything works fine as regards the payments etc. But i keep in getting these emails:

Subject - PayPal Instant Payment Notification disabled

Hi XYZ.. ,
Please check your server that handles PayPal Instant Payment Notification (IPN) messages. Messages sent to the following URL(s) aren't being received:

https://www.XXXXXx.com/ipn_main_handler.php

If you don't recognise 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.

Once you or your service provider fix this problem, you or your service provider can resend the failed messages from the IPN History page. If this problem continues, PayPal may disable the IPN feature for your account.

Thank you for helping resolve this issue.

Can anyone help me fix this issue?
Thank you