Meaning
A closed mailbox, the kind mounted on a post outside a house, with its little flag raised. A raised flag traditionally signals that mail is waiting, either to be picked up or just delivered.
It is used as a playful you’ve-got-mail signal and in contact sections: “Check your 📫 I sent you something special.” It belongs to a set of four mailbox emoji that differ only in whether the lid is open and the flag up or down.