Using URL Rewriting Page not showing 404 error message in asp.net

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ASP.NET

I am using URL Rewrite in my application.It working but when user enter wrong page name in URL which is not exit in application it should show error message that "Page Not Found". Below is my code

protected void Application_BeginRequest(object sender, EventArgs e)
       {
           // If the requested file exists
           if (File.Exists(Request.PhysicalPath))
           {
               // Do nothing here, just serve the file
           }
           // If the file does not exist then
           else if (!File.Exists(Request.PhysicalPath) && !Request.PhysicalPath.Contains("WebResource.axd"))
           {

               //var x = Request.FilePath.Remove(0);
               string profilename = Request.FilePath.Replace("/", string.Empty);
                 // Get the URL requested by the user
              // string sRequestedURL = Request.Path.Replace(".aspx", "");

               // You can retrieve the ID of the content from database that is
               // relevant to this requested URL (as per your business logic)
               int nId = 0;
               ////// nId = GetContentIDByPath(sRequestedURL); \\\\\

               // The ShowContents.aspx page should show contents relevant to
               // the ID that is passed here
               string sTargetURL = "~/profile.aspx?ProfileID=" + profilename;// +nId.ToString();

               // Owing to RewritePath, the user will see requested URL in the
               // address bar
               // The second argument should be false, to keep your references
               // to images, css files
               Context.RewritePath(sTargetURL, false);

           }
       }

In web config file custom error mode is on.please help me to solve my problem.

Thanks