New mail boxes are horrible!
I routinely use multiple different folders to save emails, and create sub folders up to 2 and 3 deep. The new functions are completely jacked up and keep making it a HUGE headache. I even tried to start a whole new thread thinking that would help. I usually have filters set up so that emails go straight to these boxes, but the new format would let me set up filters because the thread name was too long. I changed to main box to a shorter name, and it created a whole new thread, leaving all of my 2nd tiers on their own threads. the more I try to fix, the more headache it becomes. Is it time to just give up and switch the the icloud that I have been putting off? YAHOO makes it hard to think otherwise. At this point, Im not even sure going back to the classic will help UGH!

Changing a man folder name will orphan all the the related subfolders and make them invisible. For help with filtering emails, check out this article or post the specifics of your issue on our forums:
https://help.yahoo.com/kb/SLN15994.html
Thank you!
Mail Support