The meaning of this emoji is usually used as a curbside postbox (letterbox, mailbox) with its door latched shut and signal flag raised. Generally depicted in gray or blue with a red flag. Used for various content concerning email and postal deliveries.
Indicates in the United States that incoming mail (post) has been delivered. May also indicate that there is outgoing mail for pickup.
Compare 📪 Closed Mailbox With Lowered Flag, 📬 Open Mailbox With Raised Flag, and 📭 Open Mailbox With Lowered Flag.