Google “mail art” and you will find authorities — including Wikipedia and the Museum of Modern Art (MOMA) — saying that mail art began in the 1950s and ’60s.
“Mail art… initially developed out of what eventually became Ray Johnson’s New York Correspondence School in the 1950s and the Fluxus movement in the 1960s… The American artist Ray Johnson is considered to be the first mail artist.”
“Mail art—broadly defined as artists’ postal communication—emerged in the early 1960s from Fluxus, Nouveau Réalisme, and Conceptual art practices and expanded into a decentralized, global network… Artful correspondence has a long tradition [italics mine], but by the early 1960s artists were self-organizing into extended exchange networks.”
I find this definition confining. MOMA, at least, notes “artful correspondence,” and books like More Than Words: Illustrated Letters from the Smithsonian’s Archives of American Art (2005) by Liza Kirwin, and British Pictorial Envelopes of the 19th Century (1984) by Ritchie Bodily, Christ Jarvis and Charles Hahn, are filled with earlier examples of art in the mail — if not “Mail Art” then at least “mailed art” — from artists and gifted amateurs alike. And there are some fabulous examples of art in the mail in “Vintage Envelope Art Was Awesome” at Messy Nessy Chic.
A selection of mail art from British Pictorial Envelopes of the 19th Century