A post office box full of mail. PO boxes of various sizes in a German post office, with their number range and postcodes written above them. A post-office box or post office box (commonly referred to as a PO box or a postal box) is a uniquely addressable lockable box located on the premises of a post office station.In many regions, particularly in Africa and the Middle East, there is no 'door to door' delivery of mail.or 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, Egypt and Lebanon, do have mail home delivery.