Currently, the popup notifications spat out from xfce4-notifyd do not set the override-redirect attribute on those windows. Running xfce4-notifyd outside of XFCE; such as within another window manger, currently will force the notification windows to be subject to the window manager's window placement algorithm. This is counter-intuitive to the option of being able to specify where the popup notifications should then appear, for example, since setting "bottom right" has no effect if the window manger is still able to place the window.
Added spport for _NET_WM_WINDOW_TYPE_NOTIFICATION directly. Note that it wouldn't hurt to add an explicit set_override_redirect() call for those window manager which are not EWMH-aware anyway.