W2K DNS can't ping hostnames

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I have a W2K domain controller for my intranet at work. I use DHCP and
Dynamic DNS to update the zone files for the DSN server, all running
oin the W2K machine. I also have an internal website and I setup all
the workstations to use an internal DNS name for the mail server. I
switched all the workstations over from mail.mycompanydomain.com to
mail.mcd.int, which is the internal domain name. It was all working
fine for a couple days, then it wouldn't resolve any more for some
reason. I tried to ping mail.mcd.int, but it gave me a "Unknown host
name" error. I can however, perform a forward and reverse lookup using
nslookup and the results are perfectly fine, I just can't ping the
hostname. I can however, ping "mail" as a host name, and when I enter
"mail" as my smtp/pop3 server names, it works fine. Though it's not
entirely important, as I can just use "mail" for the mail server
hostnames, I can't figure out why I cannot ping the "mail.mcd.int"
hostname, but I can perform reverse and forward lookups just fine.

any suggestions?