Frequent error codes returned by NAST nameserver checker:

Error 901 - Nameserver refuses to answer
Solution: check that the zone actually exists on the nameserver that was assigned to a domain 

Error 902 - Domain update failed. Timeout (target)
Solution: check that the nameservers are online and respond to requests

Error 903 -Timeout with recursive resolver
Solution: Internal Timeout – Please try again

Error 904 - Port is unreachable
Solution: Target not listening at UDP Port 53, make sure port is setup and opened

Error 999 - Unexpected exception
This code means that the DNS query failed.
Solution : Please contact Business Services. (NOTE: NS was not accessible over UDP/TCP on port 53)

Error 110 - SOA ‘retry’ value is less than the recommended one
Solution: increase the retry value of the SOA record (4)

Error 118 - nameserver entries (NS records) in the DNS entry for the corresponding domain do not match the nameservers specified for the domain
Solution: either the zone file or the entries need to be adjusted