Urgent !!! Urgent !! named bad file number error

[Levent]

Hi to All !
We have a Sun Sparc station 5 (Solaris 2.5.1) as a Primary DNS server
and SMTP/POP3 on it .
Last sunday, suddenly DNS stopped and connection to mail ports blocked.
I've tried to telnet SMTP and POP3 ports with IP address and port
number. Ports were opening but there were no further response
from ports (no version text was returning by sendmail or popd). There
were just open ports there.
I've gone their and checked  the console and logs there was just a
message as follows that repeated more than 100 times:

    named[xxxx]: bad file number


100 times of this message was in log files.  I was be able to restart
DNS by rebooting the server.

What caused the problem ? and How can I fix this problem?

thanks