Paypal Express IPN Warnings

After several lost orders using Paypal Standard, I have switched over the weekend to Paypal Express. Since then I have received an 2 emails from Paypal saying below:

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

http://www.mywebsite.com/ipn_main_handler.php

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."

The orders are going through though but you can see from the IPN report that it retries before going through. Also I found several abandoned carts this morning and a Paypal Express warning that Address is empty. I only require Name, email and country when customers register so this could be the culprit.

Anyhow, I'm not sure if there is anything I should be doing differently on the settings? Is this a server issue?

I have since then gone back to Paypal website standard and turned Paypal debug on to monitor.

Any insight on above will be much appreciated. Thanks