why include an answer in the query?

[Kent Tong]

Hi,

We have a local DNS server that uses the ISP's DNS server 
as a forwarder. Usually it is working fine. But recently 
we find that it can't  resolve www.yahoo.com (but yahoo.com 
is OK), because the ISP's DNS server is not responding to 
the query on www.yahoo.com (but does respond to a queries 
on yahoo.com). However, using the "host" command to query 
the ISP's DNS server directly works fine for both domain
names. So Yahoo is working and the ISP's DNS server is also 
working fine.

Using tcpdump we find that if the query is for www.yahoo.com, 
our local DNS server will include an answer in the query 
to the ISP's DNS server (tcpdump shows the [1au] flag for 
the DNS packet), while for yahoo.com it won't:

[root at cladmr003 root]# tcpdump -A -i eth2 port 53
tcpdump: verbose output suppressed, use -v or -vv for full 
protocol decode listening on eth2, link-type EN10MB (Ethernet), 
capture size 96 bytes
14:54:55.723957 IP n8z108l98.broadband.ctm.net.1751 > 
macau.ctm.net.domain: 34796+ [1au] A? www.yahoo.akadns.net. (49)

E..M.. at .@.5...lb.......5.9...............www.yahoo.akadns.net...
....)........

Do you think it is this answer that is causing the ISP's DNS 
server to reject the query? If no, what else could be causing 
this behavior?

Thanks!