Users ask how they can delete all the messages in their Inbox (or other folder) without having to do them one at a time, or 20 at a time (default page size).
Click the Folders link on the left, and on that page you can delete all the messages in any folder with one click. Deleting from that page does NOT move messages to the Trash, it deletes them permanently, so use it carefully.
You can also change the messages-per-page setting under Preferences / Web Mail Options and set it as high as 200. Then select all messages on a page at once by using the topmost checkbox. If there is one you want to leave alone, just uncheck it. You can delete or move all checked messages at one time. Note that a higher messages-per-page setting will result in slower loading.