This may have been answered at some point in the past but I myself am new here and the gods know where I might find the post that answers this. It could have been years ago..
When the envelope, not what's inside the envelope, is the piece of art, does one leave it sealed with nothing inside so as not to tear it open and ruin the art, or do you put something like a note inside anyway?
Sincerely wanting to know,
The mailing itself is the art. So envelope and content and process. There are no limitations. The message is the medium. Hope this makes sence to you.
Kalimera Twizzle, the envelope is mail art whether it has something inside or not.
There are mail artists who send only envelopes, and add "DO NOT OPEN" on the outside.
Valentine Mark Herman in Sigean, France did so back a few years ago: