An open, curbside mailbox (letterbox, postbox) with its signal flag raised and a letter in its open slot. Generally depicted in blue or gray with a red flag and white envelope (Google's design is yellow). Used for various content concerning postal deliveries and email.
Indicates in the United States that there is outgoing mail (post) for pickup. May also indicate that incoming mail has been delivered.
Compare 📫 Closed Mailbox With Raised Flag, 📪 Closed Mailbox With Lowered Flag, and 📭 Open Mailbox With Lowered Flag.