When in preferences, one goes to the list of checkbuttons for which headers to show, it is possible to uncheck all of them, but when this is the case, the UI reverts to displaying at least "From", "Reply to", "To", "Subject", and "Date".

It should be possible to hide all of the headers - this is a reasonable setting because the basic info such as from/subject is usually shown in the list of messages as well.

If there is some reason that can't be done, then the preferences dialog should at least be consistent with the result, and prevent the user from unchecking all boxes.