an effective to way to test dns is to
use nslookup
the syntax is
nslookup domainname or ip address
examlpe is
nslookup
www.yourdomainname.com
this will query yourdomainname dns and return an ip address.
or nslookup (ip address of your dns)
this will query yourdomainname dns using an ip address and return full domainname
dns is important for web base mail without it you will not be able to received or sent email outside your network or domain.