Currently, when the user's account is reaching the server size quota, all the folders in the account in KMail will turn red without any explanation what that means which is super confusing to users. Even worse, if the underlying resource is in "Broken" state, it also turns the folders red, so it's even more confusing to figure out what happens.
The idea is to instead show a warning next to the account name, something like this (but better looking):
Ideally, the VDG should be included in the discussion what would be the best way to indicate that the quota threshold was reached.
The threshold for when the warning should be shown is configurable in Settings -> Configure KMail -> Appearance -> Colors -> Quota threshold. Already now this does not make much sense, but once the color will have absolutely no factor in showing the quota has been reached, a new place in the settings dialog must be found for the option.
Currently the "red color" is handled by the QuotaProxyModel, so this task just needs adjusting the proxy model to change the DisplayRole and DecorativeRole of the top-level collection that has reached the threshold.