Count and show the new mails of subfolders
Count the all mails without read in folders and subfolder and show them on mail image in the top.
Peter Hodoši commented
It would be nice if the functionality is implemented in a way, that a node of a folder tree displays sum of unread emails at its level plus number of unread emails of all child nodes (subfolders) - recursively. So in a case of complex mail folder hierarchy and automatic delivery of emails to particular folder via rules the user knows, that he/she has unread emails and visualy navigate to it through number of undread emails from top of the structure to bottom.