Такая проблема. Есть следующий код

                HttpWebRequest httpRequest = (HttpWebRequest)WebRequest.Create(this.__menuUrl);
                HttpWebResponse httpResponse = (HttpWebResponse)httpRequest.GetResponse();
                Stream responseStream = httpResponse.GetResponseStream();

На рабочей машине все нормально работает, происходит коннект к указанному сайту и получение html в responseStream
Если же сайт опубликован на сервере (на том же сервере стоит ISA Server), то при попытке соединения по указанному Url получаем следующее:

A socket operation was attempted to an unreachable host
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Net.Sockets.SocketException: A socket operation was attempted to an unreachable host

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[SocketException (0x2751): A socket operation was attempted to an unreachable host]
System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress) +1002130
System.Net.Sockets.Socket.InternalConnect(EndPoint remoteEP) +33
System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state, IAsyncResult asyncResult, Int32 timeout, Exception& exception) +431

[WebException: Unable to connect to the remote server]
System.Net.HttpWebRequest.GetResponse() +1501995
MenuParser.MenuParser.Start() in D:\Projects\Dinners (2)\MenuParser\MenuParser.cs:177
Dinners_Import1.Page_Load(Object sender, EventArgs e) +124
System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +15
System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +34
System.Web.UI.Control.OnLoad(EventArgs e) +99
System.Web.UI.Control.LoadRecursive() +47
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1061