This is not a workable solution.
I need to adjust the global text size up some, because I have trouble reading the small font in the body of email.
So I can set the global setting small, which makes the stock app work, or I can set it a little larger than standard, which makes email readable, but makes the stock app go completely funky.
Perhaps Apple should put some more thought into how this should work. When a person needs a little bit bigger type, its the SMALL FONTS that need to be increased, not the giant ones used in the stock app. Either they need the adjustment to be not the same delta for every font size, or else they need to put font size adjustments into each app.
Before Ios11, I could adjust the font size up a bit, and the stock app wasn't harmfully affected.