OK so after 6 months I have solved this problem!
It was several things, the other server (A webmethods Integration Server) is a little picky about what kinds of protocols it accepts. So after some fiddling around here are the two things that fixed it:
Servicepointmanager. securityprotocol = securityprotocoltype. ssl3;
And
Req. protocolversion = httpversion. version10;
So the whole code is:
Servicepointmanager. expect100continue = true;
Servicepointmanager. securityprotocol = securityprotocoltype. ssl3;
Httpwebrequest Req = (httpwebrequest) webrequest. Create ("https: // websiteurl: 9000 ");
// Header settings
Req. method = "Post"; // POST method
Req. contenttype = "text/XML"; // content type
Req. keepalive = false;
Req. protocolversion = httpversion. version10;
// Certificate with Private Key
X509certificate2 Cert = new x509certificate2 ("Cert. Der", "password ");
Req. clientcertificates. Add (CERT );
Req. preauthenticate = true;
String xml = "Test message" // reader. readtoend ();
Byte [] buffer = encoding. ASCII. getbytes (XML );
Req. contentlength = buffer. length;
// Wrap the request stream with a text-based writer
Stream writer = Req. getrequeststream ();
// Write the XML text into the stream
Writer. Write (buffer, 0, buffer. Length );
Writer. Close ();
Webresponse RSp = Req. getresponse ();
Streamreader responsestream = new streamreader (RSP. getresponsestream ());
Hope this helps someone else in the future.
