Active Directory Authentication

using Microsoft.Exchange.WebServices.Data;
public static bool ValidateUser(string userName, string password)
{
  bool _ExchangeConnected = false;
  try
  {
   String domain = "xy";
   String exchangeURL = "url";
   String email = "aaaa.abc@mail.com";
   userName = String.Format("{0}\\{1}", domain, userName);
   ExchangeService exchangeService = new ExchangeService(ExchangeVersion.Exchange2010_SP1);
   exchangeService.Credentials = new WebCredentials(userName, password);
   exchangeService.Url = new Uri(exchangeURL);
   exchangeService.KeepAlive = true;
   Mailbox mailbox = new Mailbox(email);
   FolderId folder = new FolderId(WellKnownFolderName.Inbox, mailbox);
   ItemView view = new ItemView(1);
   FindItemsResults<Item> items = exchangeService.FindItems(folder, view);
   _ExchangeConnected = true;
  }
  catch (Exception ex)
  {
   Logger.CreateLog.LogInfo(ex);
   _ExchangeConnected = false;
  }
  return _ExchangeConnected;
 }