How to Set the Default Message Format in Outlook
HTML is the default message format in Outlook. While plain text format works for all email programs, it doesn’t support text formatting. Rich Text Format (RTF) is only supported by Microsoft Exchange Client versions 4.0 and 5.0 and Outlook.
To configure the default format for new emails in Outlook:
Go to File > Options. In the Outlook Options dialog box, select Mail. Select the Compose messages in this format drop-down arrow and choose the format you want to use as the default for new emails. Select OK.
