Proxy authentication in a winform

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C#

.NET

WinForms

network

proxy

Hello

I'm trying to build an application that checks the availiability of some internet addresses. Therefor, i'm doing a webrequest. I need to use a proxy server on which the user, under which the application is running, isnt allowed. Thus i'm setting the proxy credentials. Unfortunately I always get a "407 proxy authentication required" error. When I run the application under an user that is allowed on the proxy, it works finde. It seems that the credentials wouldnt work but i have no idea what else to do?
Hopefully someone can help me with the problem!

HttpWebRequest request = (HttpWebRequest)WebRequest.Create("address");  
var proxy = new WebProxy("proxy", 8080);
proxy.Credentials = new System.Net.NetworkCredential("user", "password", "doamin");
request.Proxy = proxy;
request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50215)";
request.Method = "GET";
request.ContentType = "text/html";
using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
{
    //code
}

kind regards and thanks in advance!