KMail annoyingly collapses past days so you can't see the messages received those days unless you expand them. Of course this highly irritating if you have a number of past days in your inbox and you want to view all the messages quickly in your inbox. Is there any way to get KMail to automatically expand the days, or to not even sort messages under collapsable day categories?

Ah, figured it out: change View -> Message List -> Aggregation setting.