The Mailbox SP is one of the special items that Mario carries around in Paper Mario: The Thousand-Year Door. He would obtain this Game Boy Advance SP look alike at the start of the game and it would be used to receive emails though the player cannot send emails. He would receive emails throughout the game from Princess Peach, and various other people that he has helped from the Trouble Center.
When Mario gets an email, the Mailbox SP will have a ringer like the theme from Super Mario World if it is Princess Peach. Though if he gets it from anyone else, the ring tone comes from the Ice Land theme from Super Mario Bros. 3.