I recently upgraded from kde-apps/kmail-17.08.3 to kde-apps/kmail-17.12.3. Since then I have been looking for the message structure panel (giving the choice to view selected emails as HTML, if applicable). I wondered why it had suddenly gone, and I was looking for the appropriate setting.
Searching online, I found a copy of the kmail handbook of an older version (2013-12-12 KDE 4.12) which describes exactly what seems to have disappeared since kmail 17.12:
3.1 The Main Window