Emptying the Trash Folder
What I wrote below was provided in the feedback section about a but in the old and new Yahoo mail. I don't know why it's being posted as an idea:
In both the old Yahoo mail and the new one which I just started to use, if I try to delete multiple emails in the Trash Folder, north of a few thousand, it gives me the option to delete all and shows the number of messages. On numerous occasions (again with the old and the new email) I have asked it to delete all and it proceeds to do so and even shows a progress bar indicating by % points how many emails it's deleting and yet when it is done, all the emails that were supposed to be deleted are still in the Trash Folder. Signing in and out, or killing Yahoo mail and loading it again does not change anything. The highest number of emails I can delete manually is less than 5,000 or so.

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Yes, we recommend deleting less emails at one time. Seems the amount of emails you are deleting are backlogging the backend. Therefore it's stuck and taking awhile to delete.