In 1897, in an obscure Egyptian city, Oxyrhynchus, a collection of ancient Egyptian letters, mainly of business communications, were uncovered. The discovery created a revolution in modern thinking about business communication theory and epistolographic history.
Egyptian writing was a simplified and hand-written form (hieratic) version hieroglyphics. The letters were either written on papyrus or thin sheets of white stone.
Read about it among other communication history here on page 29:
http://www.engl.unt.edu/~kjensen/practice/jaconline/archives/vol9/hagge-ties.pdf
If you have the time and the resources, your library may be able to borrow a copy of "Egyptian Epistolography from the Eighteenth (18th) to the Twenty-first (21st) Dynasty"
(Bakir, 'Abd El-Mohsen.), which goes into much more detail about the collection of those letters.