Name Server Redundancy
Checks that the domain has at least two name servers defined, for redundancy.
A name server in the DNS system is a server that translates domain names that humans understand (example.com) into IP addresses that computers understand (192.0.2.128). Without a functioning name server, when someone types a domain name into their browser's URL bar, the browser will not know where to find the website.
There are many reasons why a name sever could fail. There could be a hardware failure, a network interruption, or a Denial of Service attack. Therefore, it is very important to have redundancy to avoid a single point of failure. All domain names should have at least two name severs setup in geographically different areas, to ensure that a user can always lookup the IP address.