irrelevant issue from the original post... but when you take the screen shot, there will be a tool bar that pops up... there select the three dots connected by a line icon, and there select your email app.
I don't think there is a feature to disable the screenshot ability... at least none that I know of.
although, you can change what initiates the screenshot. normally it's a swipe of the screen with the palm of your hand or the blade(side) of your hand... or Vol Down + Power key. You can turn off the palm swipe ability... also if you have any one hand mode enabled, it also contains quick screenshot shortcuts... if you have that enabled, you can change those too.