Display all received E-mails in plain text
To set Outlook to automatically display all e-mails in plain text, follow these steps:
1. Select Tools -> Options... to display the Options dialog box, choose the Preferences tab.
2. Click the E-mail Options... button.

3. To ensure that all e-mail is displayed in plain text, place a check in both the Read all standard email in plain text and Read all digitally signed mail in plain text options.
