We are currently having problems with our zimbra email server where-by all of a sudden cannot send emails out coming back with an error rejected address username@googlemail.com. Can get emails in no bother! I have just ran a nslookup on my machine and this is the output

Code:
***Can't find server name for address 192.168.8.8: Non-existent domain
***Can't find server name for address 192.168.8.1: Non-existent domain
*** Default servers are not available
Default Server: UnKnown
Address: 192.168.8.8
I have looked in our reverse lookup zones in DNS and all thats there is two Name Servers and a SOA for the server I was on. Is there sumthing wrong with our DNS?