Open mailbox with lowered flag emoji 📭 is part of the Objects and Mail emojis. On some systems it can be accessed by using :open_mailbox_with_lowered_flag: or :mailbox_with_no_mail: shortcodes.
It was approved as part of Unicode in 2014 so it should be displayed properly on all devices.
Tags: Lowered, Mail, Mailbox, Open and Postbox