Describe the envelope in early mesopotamia

One of the greatest inventions of Mesopotamian Civilization was the art of writing. 
The earliest tablets were economic records and vocabulary lists for training scribes. Later, writing was used for legal transactions, receipts, historical records such as lists of kings, sacred tests, and works of literature. 
Letters were sent by wrapping a tablet in a clay envelope, sealing it with a cylinder seal, and having it delivered by a messenger. 
5000 year old cylinder seal The cylinder seal was your signature. 
Mastering the art of cuneiform writing was not an easy task. 
The most famous writing of Mesopotamia was THE GILGAMESH EPIC.