This signed message was not sent in plain text even though Outlook is currently configured to view messages in plain text ...

This signed message was not sent in plain text even though Outlook is currently configured to view messages in plain text only. The plain text feature invalidates the signature on a signed 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".