C#错误401-用户名或密码无效

我正在尝试向API发出请求。当我用邮递员提出请求时，它是有效的。但在c#中，它没有，它返回错误401。我检查了登录名和密码是否正确。有人能帮我吗？

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代码：

public JsonResult Test()
{
    var document = new HtmlAgilityPack.HtmlDocument();
    var password = CalculateMD5Hash("******");
    var httpRequest =(HttpWebRequest)WebRequest.Create(string.Format("https://api.akna.com.br/emkt/int/integracao.php?User={0}&Pass={1}&XML={2}",
    "mail@mail.com",
     password,
    @"<main>
    <emkt trans='19.10'>
    <datainicial>2016-07-01 10:00:00</datainicial>
    <datafinal>2016-07-02 10:00:00</datafinal>
    </emkt>
    </main>"));
    httpRequest.Method = "POST";
    httpRequest.ContentType = "application/x-www-form-urlencoded";
    try
    {
    var httpWebResponse = (HttpWebResponse)httpRequest.GetResponse();
    var responseStream = httpWebResponse.GetResponseStream();
    if (responseStream != null)
    {
     var sr = new StreamReader(responseStream, Encoding.Default);
     document.LoadHtml(sr.ReadToEnd());
   }
   return Json(new { StatusCode = 200 }, JsonRequestBehavior.AllowGet);
   }
  catch (Exception ex)
  {
  return Json(new { StatusCode = 500, Erro = ex.Message }, JsonRequestBehavior.AllowGet);
 }
 }