Domain Mapping not working for me – Multisite

I setup a multisite network and added couple of test sites to it. Then I added the Domain Mapping plugin and configured it accordingly.

Now, when I open my Site A and go to tools, try to map an existing domain, I keep getting the error message of “Domain name is unavailable for access”. But, URL continues to work fine.

I have a dedicated IP and have configured it too.

Please help in this setup.