It is also a sackful of contradictions. Having originated in art-based subversion - American and European Fluxus collaborators used the post to exchange artistically embellished reportage of their outrageous happenings - it is now a vehicle not only for agit-prop and jokes but for philosophical wrangles about the nature of art (yawn) or about whether mail artists who make names for themselves are elitists. I suppose any universal communications system with popular access risks getting overrun by such anarcho-anorak tosh. Look at the Internet.
This exhibition shows that names are in the ascendant. Even the collection of anonymous fax art - office jokes such as "The floggings will continue until morale has improved" - was not gathered at random but submitted by a movement name, Alan Kane. He achieved notoriety when stopped from installing giant fax-joke murals in a Los Angeles advertising agency designed by Frank Gehry.
I looked in vain for an exploded postal package of dozens of those scurrilous bits and bobs, signed and unsigned, made from anything from coffee filters to fish hooks, that give hours of fun to syndicate members. Is this not what mail art is about? Small-circulation works, particularly on postcards, are prominent, as if mail art has become a coterie pursuit.
The philosophising is in full spate, though, judging by the exceedingly boring correspondence that introduces the exhibition catalogue, between its curator, Andrew Patrizio, the Hayward Gallery's exhibition organiser, and Clive Phillpot, the British Council's visual arts consultant - parochialism v internationalism, that sort of thing.
Has mail art mellowed as much as these recent works suggest? I saw nothing resembling the semen-stained pages of the livre d'artiste titled "Wanks for the Memories" that the American Gerry Dreva, known as "the man who had a thousand orgasms for art", mailed to his friends. Where, too, was porn to match that of Genesis P-Orridge and Cosey Fanni Tutti? Or missives remotely similar to Pauline Smith's Hitler Fan Club, dreamed up in the Seventies to goad Chelsea borough councillors?
We do at least see a collage by Ray Johnson - founder of mail art - with his bunny-face logo. Johnson was an established pop artist from whom Warhol admitted taking his Monroe collage concept. A key participant in Fluxus, he went underground in the Sixties, founding the spoof New York Correspondence School, which clogged the mail with objects ranging from nails to live lobsters, addressed to museums and galleries.
He drowned himself two years ago, leaving a characteristically enigmatic trail of mail-art clues. A postmarked postcard with bunny logo was found at his home, saying "If you are reading this, I must be dead". The number 13, the sum of the digits in his age, 67, crops up repeatedly. The hotel room where he spent his last night - number 247, add it up - has now been locked against sightseers.
But his New York gallerist, Richard Feigen, is getting a more welcome stream of visitors. His mail art sells. At his death, Johnson had $400,000 in numerous bank accounts. I would have preferred an update on the Johnson mystery rather than the Patrizio and Phillpot Correspondence School.
Star turn of the exhibition: the Chilean Eugenio Dittborn's massive wall hangings, combining self-consciously corny mail-art images with images of a hanging, mailed out of the country in 1990 to draw attention to political repression. Worth a look.
Spacex Gallery, Exeter, to 22 March. A Hayward Touring Exhibition (details: 0171-921 0837)Reuse content