I don't know if this is possible to set in kmail or kde generally - I've looked and can't find anything obvious... There is a line between the message list and message pane that can be used to resize or redivide the space between the two. It's invisible and tiny ~1px it is a bugger to find. It would be nice if it were visible and big enough to use without having to spend a lot of time finding it. I guess this may be the fault of some other component - the desktop theme or a missing fine tune control but, for me, the only time I encounter this problem is with kmail.
Fixed in 5.6.x
(In reply to Laurent Montel from comment #1) > Fixed in 5.6.x Thanks - will enjoy this when I get to 5.6.x