Won't resolve domain but all other DNS servers do

[Michael Long]

I am admin for 3 DNS servers, all running the latest 8.2.2P5 I
believe.  We host a couple thousand domains, and for the most part,
have no problems.

However, someone brought a domain to me "yorko.com" which would not
resolve for clients using our DNS server for resolving.  When you did
a nslookup it would sit there for like 30 seconds then come back and
say it did not exist.

Now we don't host this domain, and we never have.  When I go to other
DNS servers (mindspring.com, aol, etc.) they have no trouble resolving
it.

It seems to me this should be transparent - named should go to the
root server, then to dell (the host of this domain), then come back to
us with the IP.

Why is this failing for us and not for any other dns servers?  

The only way I could fix it was to put the zone in as master on our
servers for now unti I figure out what is happening