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 -> Trust Center... to display the Trust Center dialog box, and then choose the E-mail Security tab:
2. 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.