O Paypal IPN Simulator sempre retorna INVALID, apesar da postagem estar correta

Estamos atualizando nosso IPN para usar o TLS e estamos constantemente recebendo INVALID de volta ao usar o IPN Simulator

Assim que a solicitação chega ao ouvinte, ela é registrada. Logo antes dos dados voltarem ao paypal, o URL e os dados são registrados. Esta informação está abaixo.

Parece não haver nada de errado com os dados. Eu até usei o diffmerge para identificar que não há diferenças, exceto cmd = _notify-validate &

É só que oSimulador de IPN nunca retorna válido?

O URL imediatamente antes da solicitação é:

https://www.sandbox.paypal.com/cgi-bin/webscr

Os dados lançados são

cmd=_notify-validate&payment_type=instant&payment_date=Mon May 23 2016 17:41:16 GMT 1000 (E. Australia Standard Time)&payment_status=Completed&address_status=confirmed&payer_status=verified&first_name=John&last_name=Smith&[email protected]&payer_id=TESTBUYERID01&address_name=John Smith&address_country=United States&address_country_code=US&address_zip=95131&address_state=CA&address_city=San Jose&address_street=123 any street&[email protected]&[email protected]&[email protected]&residence_country=US&item_name=something&item_number=CHIMPREWRITER-LIFE&quantity=1&shipping=3.04&tax=2.02&mc_currency=USD&mc_fee=0.44&mc_gross=139&mc_gross_1=139&txn_type=web_accept&txn_id=928133899&notify_version=2.1&custom=xyz123&invoice=abc1234&test_ipn=1&verify_sign=AFcWxV21C7fd0v3bYYYRCpSSRl31AU-9VToMcj-IcSKMfmb8nz2kgIe.

Os dados do Paypal são

payment_type=instant&payment_date=Mon May 23 2016 17:41:16 GMT 1000 (E. Australia Standard Time)&payment_status=Completed&address_status=confirmed&payer_status=verified&first_name=John&last_name=Smith&[email protected]&payer_id=TESTBUYERID01&address_name=John Smith&address_country=United States&address_country_code=US&address_zip=95131&address_state=CA&address_city=San Jose&address_street=123 any street&[email protected]&[email protected]&[email protected]&residence_country=US&item_name=something&item_number=CHIMPREWRITER-LIFE&quantity=1&shipping=3.04&tax=2.02&mc_currency=USD&mc_fee=0.44&mc_gross=139&mc_gross_1=139&txn_type=web_accept&txn_id=928133899&notify_version=2.1&custom=xyz123&invoice=abc1234&test_ipn=1&verify_sign=AFcWxV21C7fd0v3bYYYRCpSSRl31AU-9VToMcj-IcSKMfmb8nz2kgIe.

Para referência, o código é:

// This was legacy code I was trying in case there were formatting problems. The code "cmd=_notify-validate&" + _request.Form; does some URL encoding which I thought might be causing problems. Either way, we still get INVALID
string s = "cmd=_notify-validate";
foreach (string paramName in _request.Form)
{
    string paramValue = LicServiceTools.Encode(_request.Form[paramName]);
    //s = s + string.Format("&{0}={1}", paramName, paramValue);
    s = s + string.Format("&{0}={1}", paramName, _request.Form[paramName]);
}

string address = "https://www.paypal.com/cgi-bin/webscr";
if (this.useSandBox)
{
    address = "https://www.sandbox.paypal.com/cgi-bin/webscr";
}
System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(address);
req.ProtocolVersion = HttpVersion.Version11;
//Set values for the request back
req.Method = "POST";
req.ContentType = "application/x-www-form-urlencoded";

// ALSO DOESNT WORK
//string strRequest = "cmd=_notify-validate&" + _request.Form;
//req.ContentLength = strRequest.Length;

errorLogger.Info(s);
errorLogger.Info(address);

//Send the request to PayPal and get the response
StreamWriter streamOut = new StreamWriter(req.GetRequestStream(), System.Text.Encoding.ASCII);
streamOut.Write(s);
streamOut.Close();
StreamReader streamIn = new StreamReader(req.GetResponse().GetResponseStream());
string strResponse = streamIn.ReadToEnd();
streamIn.Close();

// at this point, strResponse = INVALID

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