C#远程登录hotmail

public void Connect(string username, string password)
{
// Specify the server URL.
string serverUrl = "http://services.msn.com/svcs/hotmail/httpmail.asp"; // Create the object.
xmlHttp_ = new XMLHTTP(); // Build the query.
string folderQuery = null;
folderQuery += "<?xml version='1.0'?><D:propfind xmlns:D='DAV:' ";
folderQuery += "xmlns:h='http://schemas.microsoft.com/hotmail/' ";
folderQuery += "xmlns:hm='urn:schemas:httpmail:'><D:prop><h:adbar/>";
folderQuery += "<hm:contacts/><hm:inbox/><hm:outbox/><hm:sendmsg/>";
folderQuery += "<hm:sentitems/><hm:deleteditems/><hm:drafts/>";
folderQuery += "<hm:msgfolderroot/><h:maxpoll/><h:sig/></D:prop></D:propfind>"; // Open a connection to the hotmail server.
xmlHttp_.open("PROPFIND", serverUrl, false, username, password); // Send the request.
xmlHttp_.setRequestHeader("PROPFIND", folderQuery);
xmlHttp_.send(null); // Get the response.
string folderList = xmlHttp_.responseText; // Parse the folder list.

}xmlHttp_.responseText接收到的数据是NULL，请教高手