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Tap the Mail app icon, which resembles a white envelope on a light-blue background. Tap the "Back" button in the top-left corner of the screen until the "Back" button disappears. It should be near the bottom of the page. If you don't have an "ICLOUD" section on the "Mailboxes" page, your Apple Mail account isn't linked to the Mail app. Doing so opens your iCloud Mail inbox. It's in the top-right corner of the screen. Tap an email that you want to mark as spam. It's in the bottom-left corner of the screen. A menu will appear. This option is in the menu. Doing so will immediately move the selected email to the Junk folder. It's in the top-left corner of the screen. This is a few folders below the Inbox folder. You'll find this in the top-right corner of the screen. This will empty the Junk folder.
Open Mail. Make sure that you're on the "Mailboxes" page. Scroll down to the "ICLOUD" section. Tap Inbox. Tap Edit. Select a spam email. Tap Mark. Tap Move to Junk. Tap the "Back" button. Tap Junk. Tap Edit. Tap Delete All when prompted.