What is Mail Art?
Correspondence art (Mail art)

Term applies to art sent through the post rather than displayed or sold through conventional commercial channels, encompassing a variety of media including postcards, books, images made on photocopying machines or with rubber stamps, postage stamps designed by artists, concrete poetry and other art forms generally considered marginal.

John Held Jr. The Dictionary of Art, edited by Jane Turner, Macmillan Publishers Limited 1996