I need to post an XML document from one server to another server. Can I use XMLHTTPRequest for this?
Yes, the XMLHTTP Request object can be used to send requests from one server to another, with the caveat that, for performance's sake, you should upgrade the XML Technology Preview.
The capabilities that make XMLHTTP possible are predicated upon the INET services that are installed with IE5. When Microsoft first developed the control, they were anticipating that the control would be used for client to server rather than server to server communication.