Some mail artists like to see mail art as art too. They judge it as other art, menti on famous people, make a difference between good and bad mail art. But this has nothing to do with mail art. Mail art is a direct communication between individuals. No galleries, no mu seums, no shop you can buy it in.
Mail art is exchange only.
So to get mail art you will have to send it out. Mostly the mail art that is transported all over the world ends up in some archive. If the archive is big enough the museums sometimes get interested because some mail artists actually participate in the conventional art-world too. There it is judged and it can actually be worth something to the crazy collectors. But the energy that was transported when the mail-piece was sent, that energy isn't there anymore when the mail art is archived.