If a collapsed folder contains unread messages, you are unaware unless you expand the folder first.
I suggest that either the folder name be bolded, the number of unread messages contained shown to the side, or both, if there are unread messages within.
I have actually neglected responding to several messages because I wasn’t aware the folder contained those that were unread.
Leaving all folders expanded, especially those containing many sub-folders, hinders navigation.
Thanks,
John