Have you tried adding this to your solution?
<system.net>
<defaultProxy enabled="true" useDefaultCredentials="true">
<proxy bypassonlocal="True" proxyaddress="http://webproxy:80" />
</defaultProxy>
</system.net>
or try the following
string url = “Valid URL”;
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
NetworkCredential netcredit = new NetworkCredential(“user1″, “pass”, “domain”);
req.Credentials = netcredit;
System.Net.WebProxy pry = new WebProxy(“proxyServer”,true);
pry.Credentials = netcredit;
WebRequest.DefaultWebProxy = pry;
req.Method = “HEAD”; //Does not work if this line is not used
HttpWebResponse res = (HttpWebResponse)req.GetResponse();
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
NetworkCredential netcredit = new NetworkCredential(“user1″, “pass”, “domain”);
req.Credentials = netcredit;
System.Net.WebProxy pry = new WebProxy(“proxyServer”,true);
pry.Credentials = netcredit;
req.Proxy = pry;
req.Method = “HEAD”; //Does not work if this line is not used
HttpWebResponse res = (HttpWebResponse)req.GetResponse();
People have the solution in this forum
https://groups.google.com/forum/?fromgroups=#!topic/google-maps-api/0js1jr4kqJw