Domain name is unavailable to access

I've set the ip address of my server in the plugin (though the plugin seemed to think my server had a different ip address: 208.73.211.69 - the ip address of com.com) but I've set the actual ip address and now tried to map a domain on one of the subsites but it returns the error:

Domain name is unavailable to access

and says something about checking the DNS.

The domain I'm trying to map is registered and is pointing to the right ip address. What is the plugin doing when it checks the domain? Presumably a DNS lookup, but I'm wondering if there's something in my server's setup or in its firewall that could be preventing it accessing this information.