Many mail clients will send plaintext emails by default, and only use HTML if necessary (that is, the email contains bold or other formatting). Can Outlook be configured to do so?
UPDATE: I see from the answers that my question wasn't clear. In Thunderbird, for example, if I start typing a text only email, it will automatically be sent as Plaintext. If I then add bold to it, it will automatically be sent as HTML. There's no need to change settings - Thunderbird will pick the best option. How can I tell Outlook to do it? Only use HTML where the message's body needs it.