The development of submarine-launched ballistic nuclear missiles was a game-changer in Cold War strategy. The United States wanted to subtly express to the Soviet Union that it could place its nuclear-capable Regulus missiles anywhere it wanted. On June 8, 1959, it did so by using a Regulus missile from the USS Barbero to deliver 3,000 pieces of mail.
The Smithsonian National Postal Museum describes the project. The special missile was loaded with 3,000 identical letters from the Postmaster General to President Eisenhower and other leading government officials, then launched from the Barbero toward the naval air station at Mayport, Florida 100 miles away. The missile, which had wings, landing gear, and a drag chute, landed on an airstrip and the Post Office duly delivered all of the letters.
Here's a newsreel video from the time.
-via US Naval Institute