A NXDOMAIN is Non-Existent Domain. It is a term used for the Internet domain name that is unable to be resolved using the DNS servers or domain name not yet registered. NXDOMAIN can also take place due to the network or DNS server problem...
If domain name does not exists the resolving name server should return NXDOMAIN status. For example, thisdomaindoesnotexist-checkit.com does not exist, so any query sent to my ISP resolving name server should return NXDOMAIN.