I have one user who wants to access our Zimbra Server with Outlook/ZCO through a authenticating squid http proxy.

I was pretty sure that this should work as I saw the proxy configuration dialog during the ZCO installation before. But ZCO does not seem to connect to the proxy. I doublechecked that the proxy settings are correct.

What could be the problem here? Are there known issues about authenticating proxies and ZCO?
BTW: Username and Password for the http proxy are not the same as the Zimbra Account credentials...