Forcing Mail to send plain text messages

Apple’s Mail.app sends outgoing messages in a Rich Text format, meaning you can apply bolds, underlines, italics, font choices and sizes, etc. to emails you send out. If you don’t like forcing they way others see your messages, you can tell Mail.app to default to sending plain text messages.

Select the option you want by choosing Mail -> Preferences, clicking on the Composing icon on the toolbar, and choosing Plain Text from the Message Format menu. (To change the format for an individual message, choose Format -> Make Rich Text or Format -> Make Plain Text.)

via The seven most underused Mail features | Business Center | Working Mac | Macworld.