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