In the USA, people use a closed mailbox with a raised flag to indicate that there is mail in the box that needs to be picked up by the postman.
The closed mailbox with raised flag emoji was introduced in 2010 and released as part of the Unicode 11.0 standard. This emoji can be found under the objects
category. Because the closed mailbox with raised flag emoji has been around for quite some time, it should display correctly in the majority of operating systems.