A Fleet Post Office is created to deliver mail to military vessels away from their home country. The Fleet Post Office in the United States is jointly ran by the United States Postal Service and the Department of Defense. A British pillar box The postal system is a system by which written documents typically enclosed in envelopes, and also small packages containing other matter, are delivered to destinations around the world. ... The United States Postal Service (USPS) is the United States government organization responsible for providing postal service in the United States, and it is generally referred to as the post office. ... The United States Department of Defense, abbreviated as DoD or DOD and sometimes called the Defense Department, is a civilian Cabinet organization of the United States government. ...