If user’s DNS is pointing to our server(even if they are custom dns) and the name servers being used ping to the server, then check this on windows:
Go into “services” in Manager:
Find DNS Service, try restarting it. If it fails, go to bind2/etc/named.conf and cut the entire thing except the top main ones that are related to the main server and “options”
and paste into another document. Then try to restart DNS service again. If it works, then you know something in named.conf is causing problems. Undo your cut, so its back to original, then start cutting large chunks, restarting DNS, and seeing if cutting that chunk did it. In this way, isolate the conf entry that is causing problems and remove it.