(Presumably) since the last Neon package update, kMail always shows the rich text / HTML edit tool bar, so rich text mode is enabled by default. I'm always sending my mails as plain text and want to hide this tool bar. It does not remember the "hidden" setting and I could not find a configure option to change the default. This definitely worked until a few days ago. (It probably remembered the last selection state of the option, I assume.)
*** This bug has been marked as a duplicate of bug 386660 ***