Apache DNS errors

Hi All,
We are currently running apache 2.2.6 and all is running well. Except we can no longer access our Citrix gateway internally. We type in https://gateway.citrix.com.au and get this error:-

Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request CONNECT gateway.citrix.com.au:443.

Reason: DNS lookup failure for: gateway.citrix.com.au

This has only started happening since we stopped outsourcing our proxy service to MessageLabs and built the internal Apache proxy with SurfControl.

The above address works fine externally - its just internally that is the problem.

Does anyone have any ideas on how to sort this out please??

Cheers

Paul