Code :

           SerialPort port = new SerialPort("COM19");
            port.Open();
            Stream bs = port.BaseStream;
            ObexClientSession session = new ObexClientSession(bs, UInt16.MaxValue);
            string filePath=@"d:\tmp\SyncML.xml";
            FileStream src = File.OpenRead(@"d:\tmp\SyncML.xml");

            String name = Path.GetFileName(filePath);
            StreamReader rdr = new StreamReader(src);
            //strm.Position = 0;
            string txt = rdr.ReadToEnd();

            ObexHeaderCollection headers = new ObexHeaderCollection();
            string type = "application/vnd.syncml+xml";
            byte[ typeByteArray = System.Text.Encoding.Default.GetBytes(type);
            byte[ bodyByteArray = System.Text.Encoding.Default.GetBytes(txt);
            session.Connect(ObexConstant.Target.SyncML);  //This throws exception.
            headers.Add(ObexHeaderId.Count, 1);
            headers.Add(ObexHeaderId.Name, name);
            headers.Add(ObexHeaderId.ConnectionId, session.ConnectionId);
            headers.Add(ObexHeaderId.Type, typeByteArray);
            headers.Add(ObexHeaderId.Body, bodyByteArray);

            session.Put(headers);

Can you copy and paste the exception here.

LATER: I suspect the exception is occuring on the PUT.  ObexClientSession will send the correct Connection Id so there's no need to send it, so we'll get a duplicate there.  Also the rest of the PUT is too complex. :-)  Just do:

     ... ...
     session.PutFrom(src, name, type, src.Length));

That should work... (untested!)

(It also leave the contents unmodified, rather than the implicit read-as-utf8-send-as-ansi-codepage conversion happening in the code currently)

EVEN LATER: From a quick glance at syncml_obex_v11_20020215.pdf it appears that no Name header is required, so just send null for the name parameter in the Put call.