Initially :

URL enterLoginURL = new URL(ApexConstants.entrLoginURL);
   stub = new SforceService(enterLoginURL, null);



protected void initialize(String serverURL, String sessionID)
   throws MessagingException {
  try {
   
   /**
    * Once the client application has logged in successfully, it will
    * use the results of the login call to reset the endpoint of the
    * service to the virtual server instance that is servicing your
    * organization. To do this, the client application sets the
    * ENDPOINT_ADDRESS_PROPERTY of the binding object using the URL
    * returned from the LoginResult.
    */
   /*stub._setProperty(SoapBindingStub.ENDPOINT_ADDRESS_PROPERTY,
     serverURL);
        
   /**
    * The client application now has an instance of the SoapBindingStub
    * that is pointing to the correct endpoint. Next, the client
    * application sets a persistent SOAP header (to be included on all
    * subsequent calls that are made with the SoapBindingStub) that
    * contains the valid sessionId for our login credentials. To do
    * this, the sample client application creates a new SessionHeader
    * object and set its sessionId property to the sessionId property
    * from the LoginResult object.
    */
   // Create a new session header object and add the session id
   // from the login return object
   SessionHeader sh = new SessionHeader();
   sh.setSessionId(sessionID);
   /**
    * Next, the client application calls the setHeader method of the
    * SoapBindingStub to add the header to all subsequent method calls.
    * This header will persist until the SoapBindingStub is destroyed
    * until the header is explicitly removed. The "SessionHeader"
    * parameter is the name of the header to be added.
    */
   // set the session header for subsequent call authentication
   stub.setHeader(new SforceServiceLocator().getServiceName()
     .getNamespaceURI(), "SessionHeader", sh);

  } catch (AxisFault af) {
   af.printStackTrace();
  } catch (MalformedURLException e) {
   e.printStackTrace();
  }
  if (stub != null) {
   initialized = true;
  }
 }