What that error means is that there’s an element called messageheader that the xmlport is not expecting. The messageheader element in your xml file must not be defined at the same position as the one in your xmlport. I’d have to see the whole xml file and the whole xmlport.
It looks like when you posted the XML code into the editor, you have allow HTML on, so the XML was taken as valid code. What you will need to do is to put some ignore tags around your XML: such as the [ quote ] … [ /quote ] tags like this.