Cannot resolve a internal webpage

[gunbuster363]

I am using the newest version of ubuntu
the network is ok, I can google, I can facebook.
But when I try to check my company webmail (internal), it said cannot resolve hostname

what is happening? My window vista is doing ok with the webmail.
The webmail is microsoft online web access.
Ipv6 is disabled.
Using DHCP for ip address setting and DNS server configuration

I got 2 DNS servers:
10.203.65.53
10.168.9.6

I go through the internet with a proxy
and without using that proxy, I go not internet totally.

Can anyone know what is happening? Thank you.

[gunbuster363]

The requested URL could not be retrieved

While trying to retrieve the URL: http://webmail.xxxxxxxxx.net/owa/

The following error was encountered:

Unable to determine IP address from host name for webmail.hkcsl.net
The dnsserver returned:

Name Error: The domain name does not exist.
This means that:

The cache was not able to resolve the hostname presented in the URL.
Check if the address is correct.
Your cache administrator is root.