Does this have the classic XKCD 927 problem (https://xkcd.com/927/)? Also wasn't this what XML was meant to solve (a single unified standard for interop)?
It could "work" but unless you got at least Microsoft (Exchange server, Hotmail) and Google (Gmail) to sign up then it won't go anywhere (which is largely the issue with all email momentum, Microsoft and Google have no interest so it cannot move forward).
XML is a markup to establish protocol, not a standard by itself (other than the basic syntactical standard). A standard would be something like MathML.
It could "work" but unless you got at least Microsoft (Exchange server, Hotmail) and Google (Gmail) to sign up then it won't go anywhere (which is largely the issue with all email momentum, Microsoft and Google have no interest so it cannot move forward).