There should be an option to see if a folder or subfolder has unread mails in them.
Now the notification shows only if the folder has unread mail. If that folder has subfolder with unread mails there is no notification. This makes finding unread mails harder.
A simple fix: highlight or add a notification ("!" or "+" can suffice) when a folder contains mail or if the folder contains subfolder with unread mails.
I'm attaching a simple proposal.