I agree! The folder structure already exists, but the function to see which emails are sorted into which folders is still missing. This makes the function partly useless, because you can’t see which mails are already sorted into a folder (like with the lists). This should be a basic function.
I have opened another idea for this topic: https://community.hubspot.com/t5/HubSpot-Ideas/See-which-emails-are-sorted-in-which-folders/idi-p/742603