Mark Chayka writes in with a problem picking type for Mail in macOS:
I would like to permanently change the default font setting from “Helvetica 14pt” to “Helvetica Neue Light 14pt” in my mail application so that my messages appear as this one does.
(Trust me that his message did arrive in 14-point Helvetica Neue Light.)
I suggested he change Mail preferences: in the Mail app, choose Mail > Preferences > Fonts & Colors, click the Select button next to the Message font item (which shows the currently set font and size), and then use the Fonts selector that appears to choose your new options. (You also need to be sure that in the Composing preferences you set Message Format to Rich Text, not Plain Text, or the font choice won’t be set.)
However, Mark tried this, and while changing the Message Font did switch existing email messages’ display, it didn’t automatically use those font settings when he composed a new message. I tried the same steps on my Mac, and it worked without a hitch.
Forum posts on Apple’s site and elsewhere reveal this isn’t uncommon.