This article explains how to change the message format of your Outlook emails to HTML or plain text. For example, if a recipient can't accept HTML emails, you can send their messages in plain text instead. Instructions apply to Outlook 2019 through 2010, Outlook for Microsoft 365, Outlook.com, and Outlook for Mac.
How to Change Message Format from HTML to Rich Text or Plain Text in Outlook – Office 365
You can easily change the format of your Outlook email, whether you want to send a single message, reply to or forward a message, or set a default email format.
In the message, select Reply, Reply All, or Forward.
If you're in the Reading Pane, select Pop Out to open the message in a separate window. (If you're not in the Reading Pane, you don't need to do this.)
