Delete masseg all iCloud emails in one batch

To mass delete emails in Apple Mail on an iPhone or iPad, open your inbox, tap Edit, and choose Select All in the top-left corner (if missing, use two fingers to drag-select down the list) before tapping Trash. On a Mac, simply open the mailbox, press Command + A to highlight every message, and hit the Delete key.

Remember that these actions only move items to the Trash folder; to permanently free up storage, you must navigate to the Trash bin and manually empty it by selecting Edit > Select All > Delete (iOS) or right-clicking to choose Erase Deleted Items (Mac).