Art in the form of mail. The posted package itself is the work of art and, unlike a traditional letter or parcel, it is intended to be received and admired for it’s own sake. While a normal item of mail fulfills a practical purpose such as delivering a bill, a letter from a loved one, a notice from an employer, or a gift sent by a friend, with mail art the box or envelope becomes the center of attention instead. – An excerpt from Mail Me Art: Medium Without a Message.
For an introduction to the community that surrounds Mail Art; check the International Union of Mail Artists. IUOMA