A post office box (often abbreviated P.O. Box or PO Box) is a uniquely-addressable lockable box located on the premises of a post office station.
In many countries, particularly in Africa, and the Middle East there is no 'door to door' delivery of mail. For example, should one post mail to a street address in Namibia, it will be returned to sender as undeliverable. Consequently renting a PO box has traditionally been the only way to receive mail in such countries, although some, like Jordan, are now introducing home delivery.
Generally, post office boxes are rented from the post office either by individuals or by businesses on a basis ranging from monthly to annual, and the cost of rent varies depending on the box size. Central business district or CBD PO boxes are usually more expensive than a rural PO box.
In the US, the rental rate used to be uniform across the country. Now, however, a postal facility can be in any of seven fee groups by location; in addition, certain customers qualify for free box rental.
In the United Kingdom, Royal Mail PO boxes are often little more than pigeon holes in the secure section of a Sorting Office, and are only accessible by staff. In such cases, the renter of the PO box will be issued with a card on which is written the PO box number and post office name, and must produce this to the desk staff when collecting mail. For an additional fee, the Royal Mail will deliver received items to the renter's geographical address.
Sometimes the numbering schemes of several post offices in the same suburb are combined so that, for example, 1-999 belongs to the Main Office, 1001-1999 to the North Office and so on. In many U.S. cities, the first two digits of the box number are tied to the last two digits of the ZIP code, leaving either three or four least significant digits to identify the P.O. box itself. For example, in Chicago, Illinois, P.O. boxes in the 11000 range are in ZIP code 60611, while those in the 12000 range are in ZIP code 60612, and so on.
In the United Kingdom, boxes are numbered consecutively. Each box is allocated a unique postcode, appropriate for the postal town in which the box is located.
In some countries, PO boxes are usually numbered left to right, top to bottom, from the point of view of the postal worker on the inside, so that the boxes are numbered right to left from the point of view of the box-holder.
Other countries use numbering systems different from that just described. For example, in the United States, it is typical to number PO boxes from left to right from the point of view of the box-holder. In newer facilities, the boxes are numbered from top to bottom and then from left to right from the point of view of the box-holder; the least significant digit indicates the position of the box in a column.
Post office boxes are usually mounted in a wall of the post office, either an external wall or a wall in a lobby, so that staff on the inside may deposit mail in a box, while a key holder on the outside of the building may open his or her box to empty the mail. In many post offices in the U.S., the P.O. box lobby is separate from the window-service lobby, so that the former may be kept open around the clock while the latter is locked after business hours. However, in the U.S. since the 1980s, in cities and large urban areas, post offices have tended to close box lobbies overnight because of the tendency of homeless people to use them for sleeping quarters. As a result, some box lobbies are accessible after-hours by customers who are provided a code to a door keypad.
If a parcel does not fit in a PO box, the postmaster will leave a note advising that customer to pick up that parcel from the counter. In some post offices, a key will be left in the PO box that corresponds to a larger, locked box where the patron may pick up his or her package if a signature is not required. Notes will also be left in the PO box in respect of COD and registered mail that has to be signed for.
Users receiving very large quantities of mail can use "locked bags", which are numbered like PO boxes. In the United States, this service is called caller service, and the assigned number is called a caller number, although mail is typically addressed to "P.O. Box (caller number).
In Australia, wall-mounted PO boxes come in three sizes, which are designed so that different sizes can be mixed almost completely arbitrarily on the wall. There are five sizes in the U.S., using that same design principle, although the boxes are normally arranged from the smallest boxes at the top to the largest at the bottom. The two largest sizes are configured as drawers.
In the United Kingdom, anyone applying for a Royal Mail PO box must produce documentation to verify their home or business address. The Royal Mail will provide, on request, the geographical address relating to a PO box, unless the local Police have requested that the information be withheld.