How to send non-HTML mail from Apple Mail?
The alternatives are described as Plain Text or Rich Text - however I am reliably informed that 'Rich Text' is, in fact HTML.
(Thanks to @TomGewecke for correcting me on that in comments)

Swap permanently in Prefs > Composing, or as a one-off in Format Menu, last item, or  Cmd ⌘   Shift ⇧ 
  T  

To prevent replying in HTML to an HTML mail, also uncheck 'Use the same Message format...'