Here are the instructions in Outlook 2007 for opening messages in Plain Text.

1. On the Tools menu, click Trust Center, and then click E-mail Security.
2. Under Read as Plain Text, select the Read all standard mail in plain text check box.

To include messages signed with a digital signature, select the Read all digitally signed mail in plain text check box. Note  If you want to view a plain text message in its original format, click the InfoBar, and choose either Display as HTML or Display as Rich Text.