Compose a Plain-Text Message

I want to compose a message in plain text, instead of the HTML-based rich text form offered by default. How do I do this?

To compose a message in plain text, hold down the Alt key while clicking the Compose or Reply button. The composer window will open in plain text mode, without the usual formatting toolbar.

If you want to use plain-text mode all the time, go to Preferences > General and uncheck Enable rich text and advanced editor features under the Composing section.