Created attachment 107719 [details] Before_adjustment.png After changing the width of the columns in the messages list to fit content, these settings are preserved during the day even after closing kontact or even after a reboot. However at midnight these settings are reset to some default which means that the content of the columns does not fit in the width of the column anymore. See attached screen copies Before_adjustment.png, which is what it becomes after midnight. After_adjustemnt.png shows what it should always remain.
Created attachment 107720 [details] After_adjustment.png
Can confirm the problem in KMail 5.9.1 (actually, that annoys me every day as well)
Still not solved.