DNS problem with Win2k to NT4

Hi,
We have an NT4 domain server. Every day the Win2k clients loose connectivity to the mail server, whichi is being mapped trough the domain’s dns. All the clients have static IPs, but only the Win2k machines lose connectivity. The only ways to get the Win2k clients to see the mail server is to either reboot the machine or open a command prompt and type “ipconfig /registerdns”. Any ideas as to why this is happening?

Thanks in advance,

Carlos