massarrah:

Diplomatic Letter between Ancient Babylon and Egypt

This letter written in Akkadian cuneiform is addressed by the Babylonian king Burnaburiash, a Kassite, to the Egyptian pharaoh Amenhotep IV (Akhenaten), the so-called “heretic pharaoh” who tried to bring about the abandonment of traditional polytheistic worship in favour of worship centred on the god Aten. A number of such letters from the Late Bronze Age between the Great Powers of the Ancient Near East, including Babylonian, Assyrian, Mittanian, and Hittite kings and the Egyptian pharaohs, provide some of the earliest glimpses into diplomatic correspondences and practices, such as gift-giving and political marriages. 

A collection of 382 cuneiform documents are known to have survived from this period of correspondence, for which the lingua franca was Akkadian. The documents, most of them letters written on clay tablets, were found at the Egyptian site of Tell Al-Amarna. (Source)

Tell Al-Amarna, Kassite Dynasty, c. 1359-1333 BCE.

British Museum.