The problem is the way it comes out of the box. In Apple Mail you can "simply" open Preferences > Composing and set "Message Format" to "Plain Text". There is

The problem is the way it comes out of the box. In Apple Mail you can "simply" open Preferences > Composing and set "Message Format" to "Plain Text". There is also the option for each message you write to Format > Make Plain Text But you have to know that the option exists before you can go looking for it.