Mail from Guy Bleus in Belgium in 2003. Notice the special Mail-Art stamps from Belgium (official ones) that he placed on the enveloppe.
In the 80-ies I was experimenting with selfmade artistamps. Tried them on envelopes and let them run through the system. Seems the system worked quite well. In the beginning years of mail-art I even numbered outgoing mail-art. This was enveloppe number 1000. After a few years I quit with that numbering