MX query fails

[czimmer at wczimmerman.dyndns.org]

Can some of the DNS experts chime in here?

Our zone:  mclaneco.com
Queries from 204.238.58.50 & 51 cannot get a response for an MX query
from our DNS servers.  Other name servers seem to work fine.  We put a
packet sniffer on the network yesterday and saw the reply UDP packet
going out to the above IP, but apparently it never makes it there.
We're looking into a possible network issue of some sort, but I want to
verify with the experts that we don't have something misconfigured
here.  From www.network-tools.com we get a Timed Out error when doing
the MX query on our domain.  Yet, email is working from thousands of
others.