Software Update causing proxy traffic

Hi, we have sus running on our xserve. The SU clients have been hammering our proxy server. Each time a client checks for updates, the traffic goes through the proxy. Why is this? I removed the proxy servers address from the 'Router' field of the Network pane in the system preferences. This solved that problem, and made teh updates go much faster, but causes problems with people trying to pop mail from external servers. Does any know of a better fix? l.