If a received mail contains a plain-text part I prefer Mail to display that instead of the often more “stylish” HTML or Rich-Text parts. While this cannot be configured via the GUI, it can be from the command line.
This is a reminder post for myself and anyone who might be interested.
defaults write com.apple.mail PreferPlainText -bool TRUE
defaults delete com.apple.mail PreferPlainText
Mail.app should be closed while you do this.