Hi,
I'm running kmail 4.14.10 within opensuse leap 42.1. I have my emails written by composer. I would like to change the default font and size but have failed so far. I would have thought that I could do it by 'settings-> Configure kmail->Appearance->Use Custom fonts/Apply to composer'. However, it consistently ignores my settings, even after closing down kmail and starting it again. Is this a bug?

I'm not too sure about this. It should work, in theory (I used to do so in the past). I'm not sure if it's worth reporting a bug for this, because a lot of these options were removed in the latest KDE PIM (KF5 based), due to their huge negative impact on performance.