Outlook for Windows
In Outlook, select “File” > “Options“.
Select “Mail” in the left pane.
In the “Compose messages” section, change the “Compose messages in this format:” to “HTML“, “Rich Text“, or “Plain Text” as desired.
Outlook for macOS
While in “Outlook” > “Preferences“.
Select “Composing“.
Check the ”Compose messages in HTML by default” if you want to write messages in HTML. Uncheck it to use plain text.
While composing the email, it is easy to set this instead of using default settings, Just set this in the single email you are composing,
Select “New Email” to compose the message, then select the “Format Text” tab.
In the “Format” section, select “HTML“, “Rich Text“, or “Plain Text” as desired.
If you are missing the “Format Text” tab in Outlook for Windows, select “File” > “Options” > “Customize Ribbon“. Choose “Main Tabs” in the “Customize Ribbon” dropdown, then ensure the “Format Text” option is selected.