I am a fan of plain text, so I prefer using plain text mails whenever possible. Here is how you do it for Apple Mail ( OS X El Capitan, but probably works on most versions).
Change the (default) format for all messages:
- go to Mail > Preferences, click Composing, then choose Plain Text as Message Format.
Change format of the current message:
- go to Format > Make Rich Text > Make Plain Text
Reply to messages in the same format you received them:
- go to Mail > Preferences, click Composing, then select Use the same message format as the original message
Most probably you will also want to use fixed-width font for plain text messages. So, go to Mail > Preferences > Fonts & Colors and choose Use fixed-width font for plain text messages