Excerpt from Wikipedia.
Mail art is art which uses the postal system as a medium.
Mail art is also, simultaneously, a message that is sent, the medium through which it is sent as well as one of the longest-lasting art movements in history. To be precise, an amorphous international mail art network evolved of thousands of participants in over fifty countries between the 1950s and the 1990s from the work of Ray Johnson and influenced by earlier groups, including Dada and Johnson's contemporaries in the Fluxus group. A theme involved in mail art is that of commerce-free exchange; early mail art was, in part, a snub of gallery art, juried shows, and exclusivity in art. A premise of mail art is that "senders receive," meaning that one must not expect mail art to be sent to them unless they are also actively participating in the movement.
Mail artists characteristically exchange ephemera in the form of illustrated letters, zines, rubberstamped, decorated or illustrated envelopes, artist trading cards, postcards, 'artistamps', mail-interviews and three-dimensional objects.
Whether one is a formal mail artist, there exists a rich history of creative examples sent through the post to draw upon. The most familiar example is the illustrations on envelopes carrying first day issue postage stamps, which philatelists refer to as first day covers, but mail art encompasses other "decorated envelopes" as well as a wide range of other procedures and media such as rubberstamping and the creation of artistamps. Mail art is traditionally, though not always, distinguished from simply "mailed art," which is art that does not truly use the postal service but is simply regular art when sent through the mail.