Deleting trash: is there really no way to quickly empty trash 100%, eg double confirmation? It's torture to delete 100 at a time.
Why not give users the option to empty trash 100% - instead of 100 emails at a time - say via a double confirmation message? Even a triple confirmation is significantly better than constraining to 100 emails at a time. You need to make Yahoo mail more efficient for users to go back to using it as their main email.

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Minette Navarrete commented
Also: when we mark ALL messages, and then we click the TRASH icon, we actually mean ALL messages, NOT 50 messages. You have to imagine the user experience of cleaning up an overfull inbox: even after finding all the messages, you can only transfer from Inbox to Trash 50 emails at a time. Then you can only permanently delete from Trash 100 emails at a time. It's terrible UI/UX to not offer the option, and worse yet to pretend the "ALL" option exists, but restrict it to 50 or 100 emails at a time.