New Mail Alert Ball prevents wasted trips to the mailbox. This mailbox alert has a bright yellow ball that pops up to signal “mail call” from hundreds of feet away! It’s the easy mail sign you asked for ... a spring-loaded ball pops into vertical position when postal carrier opens your mailbox, eliminating guesswork. Installs easily to most rural mailboxes; hardware and instructions included. Metal, plastic. 6 1/2" high.