DNS error through corporate proxy

Hi,

I run a website on multiple IIS servers, referencing multiple MS SQL servers. Several customers have complained that when they try to access the site via thir corporate proxy they get DNS errors or timeouts after logging it to the site. We have had these problems with Wingate amd MS Proxy server users. I set up a WinGate server on our network, but I could not reproduce the problem. Does anyone know if there are settings on the Proxy that can be changed, or even on our own servers?
Many thanks!