When starting KMail for the first time, many splitters have non-useful default values, especially the main splitter between the folder and the message list. Also, the folder list is affected. The folder list should not show all available columns, only "total", to save space. The column sizes in the message structure viewer are also affected.
I think this could be closed as it doesn't happen anymore. The folder list and the message list seem to be split with a ratio of 1 to 3, which seems to fine. Also, in the folder list pane, only the folder name and the "Total" columns are visible.
Yep, seems to be ok now.