It seems fair to say that a mail art piece needs to be sent through the mail system to be “mail art”. But then there is German artist Bodo Berheide (b. 1944) and his two handgribs. The handgribs were part of a piece called “Between the poles” that used the network of mail artists as a distribution system.
Another German artist Jürgen Olbrich mailed a handgrib to Mogens Otto Nielsen I 1985 with a message from Berheide: “he wants that this grib gets as close as possible to the northpole. It doesn’t matter how quick it goes, just send it further north to a person that you think will do the same.” The handgrib was mailed from West Germany to Denmark, but possibly just passed on from person to person from there. The important thing is, that the piece was a part of a distribution system or network, and it is this merging with a system that makes it mail art.
From the following correspondences we know, that the north-going handgrib went from Mogens Otto Nielsen to Upernavik in Greenland. There is little information about the south-going grib, because it didn’t go through Mogens Otto Nielsen, only that “the south-grib is lost somewhere in space of the mediterranean”, as Berheide writes in a letter in 1985.