After setting the header sizes for the columns in the message list view, and using the app for a while, over time they will resize in a way that ends up with the second-rightmost column having a not particularly useful size. Please see the attached screenshot for how things look after a while (the very narrow column ends up entirely squeezed to no width eventually, but i thought this shows the effect just as well).
Created attachment 135210 [details] KMail message list column headers after a while
Looks similar to https://bugs.kde.org/show_bug.cgi?id=430626