Mail.app Won't Change Message Font Size macOS Sierra 10.12.6, Mail.app 10.3, 

I can change the size of the message list font in Mail > Preferences > Fonts &amp; Colors, and the change is reflected in Mail.app's message list. However, changes to the message font are not reflected by a change in the message display. 

This behavior appears on two computers, a Macbook Pro 7,1 (mid-2010) and a Macbook Air 6,1 (mid-2013). Both computers' software is updated. In all other respects, both computers operate correctly.

Is there a cache or plist to clear or something else to do to allow the message font size to change?
Reviewing this thread in Apple community discussions, it seems that:

a) incoming email font/color/size is determined by what the sender specified;

b) the receiver's message list display is determined by what's set in the receiver's Mail > Preferences > Fonts &amp; Colors > Message List Font; and

c) the receiver's reply (or new message) font/color/size is determined by sender's settings in Mail > Preferences > Fonts &amp; Colors > Message Font

What I'm looking for — that a received message be displayed in a font larger than what's specified by the sender — is apparently not available short of Applescript or add-on app.

Sigh.