The Smithsonian National Postal Museum has published an article about the use of a Regulus surface-to-surface missile fired from a submarine in 1959 to carry a payload of mail. As the article says, it was ostensibly an experiment in communication transportation, the Regulus’ mail flight sent a subtle signal that in the midst of the Cold War, the US military was capable of such accuracy in missile flight that it could be considered for use by the post office.
https://postalmuseum.si.edu/collections/object-spotlight/regulus-mail.html
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