What is the expected output? What do you see instead? The expected output is just space seperated output that is hard to parse by another application. If MU would output XML data the data can be used elsewhere more easily. Such as in a web page, creating a powerful online search app that searches your mail.

What version of the product are you using? On what operating system? 0.7, Ubuntu

Please provide any additional information below. The standard C XML library is located here: http://xmlsoft.org/ to write XML

is there a standard xml schema for such data?

No, or at least, I didn't come across such a shema.

latest git version has --format=xml, still experimental.

there's (somewhat experimental) support for xml/json/s-exp in mu 0.9.2.