This signed message was sent in plain text. Your administrator has specified that all messages must be viewed in plain text ...

This signed message was sent in plain text. Your administrator has specified that all messages must be viewed in plain text format. As configured, Outlook cannot convert signed messages into plain text and cannot display this message. To read signed messages in plain text without disabling signatures, on the Tools menu, click Trust Center, click E-mail Security and then select "Read all digitially signed mail in plain text".