Calling writeTo on a SoapMessage from within a JAX-WS handler handling an incoming message the following exception is thrown: -

16:17:18,230 ERROR [STDERR] java.lang.IllegalStateException: XOP parameter not properly inlined
16:17:18,231 ERROR [STDERR] at org.jboss.ws.core.soap.SOAPMessageImpl.saveChanges(SOAPMessageImpl.java:253)
16:17:18,231 ERROR [STDERR] at org.jboss.ws.core.soap.SOAPMessageImpl.writeTo(SOAPMessageImpl.java:309)
16:17:18,231 ERROR [STDERR] at org.jboss.support.ws.handler.CustomHandler.handleMessage(CustomHandler.java:35)
16:17:18,231 ERROR [STDERR] at org.jboss.ws.core.jaxws.handler.HandlerChainExecutor.handleMessage(HandlerChainExecutor.java:305)
16:17:18,231 ERROR [STDERR] at org.jboss.ws.core.jaxws.handler.HandlerChainExecutor.handleMessage(HandlerChainExecutor.java:142)
16:17:18,231 ERROR [STDERR] at org.jboss.ws.core.jaxws.handler.HandlerDelegateJAXWS.callRequestHandlerChain(HandlerDelegateJAXWS.java:97)
16:17:18,231 ERROR [STDERR] at org.jboss.ws.core.server.ServiceEndpointInvoker.callRequestHandlerChain(ServiceEndpointInvoker.java:125)
16:17:18,231 ERROR [STDERR] at org.jboss.ws.core.server.ServiceEndpointInvoker.invoke(ServiceEndpointInvoker.java:201)

This can be resolved by moving the following two lines within the ServiceEndpointInvoker to above the handler calls: -

if (binding instanceof CommonSOAPBinding)
XOPContext.setMTOMEnabled(((CommonSOAPBinding)binding).isMTOMEnabled());