The user is working at a machine that does not have a PTR record in the master server's hosts.rev file.
A missing or incorrect delegation of a subdomain in the hosts.rev file.
Diagnosis and solution
Check the appropriate hosts.rev file and make sure there is a PTR record for the user's machine. For example, if the user is working at the machine altair.doc.com with an IP address of 192.168.0.1, the doc.com master server's doc.rev file should have an entry like:
46 IN PTR altair.doc.com.
If the record is missing, add it to the hosts.rev file and then reboot the server or reload its data as explained in Forcing in.named to Reload DNS Data.
Check and correct the NS entries in the hosts.rev files and then reboot the server or reload its data as explained in Forcing in.named to Reload DNS Data.