Paypal Reporting IPN Setting Error

So, I was working on a rebuild of a site that uses the membership plugin, accessing it via the server IP address. While testing I used the IP address in the URL for the Paypal IPN settings. When I went live I changed the setting in the PayPal dashboard the use the ful domain name. All seemed well, until I got the following email from PayPal:

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

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.

I asked PayPal if they could advise further and they have confirmed that the URL with the full domain name has been entered into their dashboard correctly, and that the reference to the IP must be within the script itself.

So I'm turning to you, is there anywhere within the plugin this data might be stored?