Outlook 2013 only displays html instead of formatted message

I found on my Outlook 2013 RTM that some of my incoming html formatted mails are displayed correclty, whereas others only appear with the html code. When I copy that code into a new html file and open it in a browser the message is displayed as desired.

The problem occurs independtly of the outgoing email client (I found this behaviour on Messages from Outlook-2007 colleagues as well as on messages from Lotus Notes)

I already tried various options, but it did not improve anything.

thanks for your help!