Unfortunately, turning off Snap View may work in Desktop Mode, but it does nothing in Tablet Mode, and that's true even in Windows 10. I've upgraded to Windows 10 and have Snap completely turned off, but in Tablet Mode, it still insists on opening Mail links in split screen. Any solution that involves running in Desktop Mode is more than useless on a tablet.
I gave my old Windows 10 tablet to my wife who has come from an Android tablet, and she told me she loves the tablet and is generally happy with Windows 10, except she hates that Mail links open up split-screen and she asked how to make them open full screen. I had to tell her, "You can't. This is something people have been complaining about since Windows 8.1".
How can this complaint be escalated to someone who can actually do something about it?