There are no touch screen mail programmes. If you're using the touch interface, you need something touch friendly. There are almost no mail apps in the store. 'Proper' mail programmes such as thunderbird are almost unusable by touch.
Added to which, the average user - someone like my sister for example - will want to use the built in app.
Oh, that's right, you're using a tablet! Bob's RT comment got right past me. Yeah, Windows 10 isn't upgrading RT devices until September. They're focusing on desktop versions right now, since that's what is releasing on the 29th.
RT devices are getting a partial update with some of the Win10 features(they are deemed incompatiable with Windows 10 by Microsoft) that's going tbe called Update 3.