If an email is not authenticated, Gmail does not make its content available for the sidebar, and the sidebar will display "Content not available for this message".
When an email is not authenticated, Gmail cannot verify the sender of the email is the one who is actually sending it. There will also be a question mark next to the sender's name.
If the message is coming from the person who appears to be sending it., or you want more information, see Gmail's article Check if your Gmail message is authenticated.