This is what I was talking about… Maybe Win 10 is different as I’m on Win 7 and with a small toolbar to maximize screen real-estate. In this environment, the open window icon also does not change so the indication of waiting mail is poor:
Still, it’s fashionable I guess… That’s what all interface designers seem to like nowadays, flat designs with no contrast!
In Windows 10 with small icons, the application icon does not display the new mail overlay, but the new mail icon in the system tray does display.
If you look closely it is there… it is just black on dark grey! 
And what happens if you change the taskbar colour?
This is also now “fixed” as the icon appears white on darker backgrounds in v7.1 as Gary demonstrated in his pic above. It is not coloured and totally flat but I that’s the fashion and at least it is visible!