Mostly new Live products from Microsoft uses the msncore library to render it's windows (also known as uxcore since the 9.0 split up), so Windows Live Mail is using the same method as Windows Live Messenger for its definition, images, styles etc.
Plus! runs inside Messenger's process so it cannot touch the Windows Live Mail process that easily, and therefore I doubt it's going to be implemented in Plus!. Mail Plus! is what you want.
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