The meaning of this emoji is usually used as a curbside postbox (letterbox, mailbox) with its door latched shut and signal flag lowered. 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 no incoming mail has been delivered. May also indicate that outgoing mail has been picked up.
Compare 📫 Closed Mailbox With Raised Flag, 📭 Open Mailbox With Lowered Flag, and 📬 Open Mailbox With Raised Flag.