This policy setting determines whether Outlook renders all digitally signed e-mail in plain text format for reading. Outlook can display e-mail messages and other items in three formats: plain text, Rich Text Format (RTF), and HTML.
If you enable this policy setting, the "Read all standard mail in plain text" check box option is selected in the "E-mail Security" section of the Trust Center and users cannot change it. This option only changes the way e-mail messages are displayed; the original message is not converted to plain text format.
If you disable or do not configure this policy setting, Outlook displays digitally signed e-mail messages in the format they were received in.