Attempting to contact the OWA site that is upstream from us returns
"part" of a 504 error page from border manager (only some of the error
screen is displayed). This only happens for this one site.

If we configure a browser to talk to the upstream proxy directly, there
are no problems.

The upstream proxy and the OWA page are hosted at the same location
with both internal and external ip addresses. Our DNS resolves the
external ip address only, whilst I assume their DNS will resolve their
internal address. I don't know if this is the cause of the problem or