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
If you prefer to view your email in HTML format instead of plain text (Default) click on the bar located on the left hand side of the pane. The text on the bar will inform you which view you are in. If this bar is missing you can enable it in the Settings → Configure KMail... dialog, from there you want the Message Window tab, and select Show HTML status bar
For complex messages you can show the structure of the message by selecting View → Show Message Structure, this will show in the message structure viewer below the preview pane.
All the UI and configuration elements mentionned in bold above seem to have disappeared.
Same but with better formatting:
*** This bug has been marked as a duplicate of bug 387931 ***