Currently there is no way to use up / down to navigate Kmail message list and message body depending on the current panel focus. It's pretty counter intuitive. Current configuration doesn't allow assigning it to both, and only one action can be configured. Can you please enable that depending on what is focused at the moment? Thanks!
Use Left/Right to navigate in the message list, and Up/Down to navigate in the message. This doesn't require you to change focus.
Using focus makes it more natural. It's unnatural to use left and right to move up and down.