Changing the format for individual emails
The email format can also be changed each time you send an email by choosing Format->Plain Text from the menu bar when you create a new email. Now send your message and it'll be sent in Plain Text.

The best option is to update specific contacts that need to receive Plain Text only. This will allow you to utilize the other formatting features offered in Outlook for all others, and prevent the need to change the formatting for each email sent to recipients that don't have an Outlook-compatible email client.