I am working on a Java Servlet to receive IPN from PayPal and post back with cmd=_notify-validate. I basically copy the JSP code sample and motify to a servlet code. But everytime I received an INVALID code after I post the url back to PayPal. I did a print out and all the parameters and parameters values are extactly the same as received along with the added parameter cmd=_notify-validate. I am testing this in the SandBox envoirnment. Can someone help me? Anyone got the Java Servlet code working that received VERIFIED code from PayPal. I really don't know what is missing. The only thing I can think of is PayPal requries all values passed back the way it is and in the same order (read it in the IPN specs help page). But I am not sure if request.getParameterNames which returns the paramter names as an Enumeration objects returns the elements in the order it was posted. Any help would be appriciated. I copy and pasted my code here, it is really simple. Just don't know why it does not work. Enumeration lEn = request.getParameterNames(); String lstr = "cmd=_notify-validate"; //get the parameters and build the return url while (lEn.hasMoreElements()) { String lParaName = (String) lEn.nextElement(); String lParaValue = request.getParameter(lParaName); lstr = lstr + "&" + lParaName + "=" + URLEncoder.encode(lParaValue); } //post back to PayPal system to validate URL lurl = new URL("https://www.paypal.com/cgi-bin/webscr"); HttpURLConnection uc = (HttpURLConnection) lurl.openConnection(); uc.setDoOutput(true); uc.setRequestMethod("POST"); uc.setRequestProperty("Content-Type", "application/x-www-form-urlencoded"); PrintWriter pw = new PrintWriter(uc.getOutputStream()); pw.println(lstr); pw.close(); //PayPal responds to the postback with VERIFIED or INVALID BufferedReader in = new BufferedReader( new InputStreamReader(uc.getInputStream())); String lResult = in.readLine(); in.close(); //check notification validation if (lResult.equals("VERIFIED")) { //log for investigation } else if (lResult.equals("INVALID")) { //this should not occured, log for investigation } else { } saveData(request, lResult, lstr);