Hello,
That means the domain name is not resolved. For example if your domain name: domain com, the following command must return something look like:
$ host domain.com
domain.com has address 216.34.94.177
domain.com mail is handled by 10 sentry.domainbank.com.
If you domain is not resolved, you get:
$ host yourdomainwhichisnotresolved.com
Host yourdomainwhichisnotresolved.com not found: 3(NXDOMAIN)
Either your dns servers do not work or external dns server do not work. To find out which of them serve your domain, run:
# whois domain.com
....
Domain servers in listed order:
NS1.DOMAIN.COM
NS2.DOMAIN.COM
NS3.DOMAIN.COM
This servers serve your domain. Try to connect to them to 53 (dns) port:
~$ telnet NS1.DOMAIN.COM 53
Trying 72.5.54.12...
Connected to NS1.DOMAIN.COM.
Escape character is '^]'.
If you connect, then it is something wrong with dns configuration. If you do not connect, so, either firewall blocks it or dns server(s) is(are) not running.