popout a mac mail window from full screen / split view
I asked Apple support about this one. The representative didn't seem to think that this was a supported feature in current OS X (El Capitan) and passed it along as a feature request.

The only other workaround I can think of is right-clicking Mail in the Dock to open a new viewer or new message on your primary display. This works for me when I have Mail in full-screen in an off-screen space. I haven't tried it on a dual-monitor setup yet.