I'm not certain what you aim is (is this MBean a service?) but have a look at mail-service.xml in your deploy/ directory. You can see in this ServiceMBean config file that XML is set as an attribute under the name Configuration:
<!-- Test -->
<!-- Change to your mail server prototocol -->
If you browse the code for the mail service, you will see that when setConfiguration is called the XML "fragment" beginning with is set to a local XML node object, then parsed to setup the contained properties.