Let me select the font I read. The apparent selection tool under Settings->General does not change what I see in the mail pane.
Suggestion 1: For each of the fonts, show the font name IN THAT FONT and a line using lots of different letters to the right.
Suggestion 2: For uninstalled fonts, let me click a button appearing just to the right of the text saying "font not installed" which button has the label "Install this font" (text shown in that font please) to install the particular font.
Suggestion 3: When I change display font in Settings, IMMEDIATELY show my current mail read pane in that font. This can be done if I've moved the Settings tab to another window/monitor.