A Seapost was a mail compartment aboard an ocean-going vessel in which international exchange mail was distributed. The first USA service of this type was the U.S.-German Seapost, which began operating in 1891 on the S.S. Havel North German Lloyd Line. This service rapidly expanded with routes to Great Britain, Central America, South America, and Asia. The Seapost service still employed fifty-five clerks in early 1941. The last route of this type (to South America) was terminated October 19, 1941, due to unsafe wartime conditions on the Atlantic Ocean. The few remaining Seapost clerks transferred to other branches of the Railway Mail Service (RMS). Seapost operations for the Post Office Department were supervised from a New York City, New York, office.
Sea Post Offices were also operated by the Postal Authorities of France, Germany, Great Britain, Italy, Japan and New Zealand.