Eusebius (265-340 AD), the Christian bishop of Caesarea in Palestine and famous father of church history, wrote in his "History of the Church" that he had seen and translated into Greek from Syriac some significant documents in the Record Office at Edessa.

Among them was a copy of a letter which Abgar Uchama wrote to Jesus, and Jesus' reply.
See Abgar V Uchama.
 

From Armenian sources quoted in "The Kingdom of Armenia" by M. Chahin we also know that King Abgars messenger not only returned with the letter quoted by Eusebius, but also brought with him a portrait of Jesus. Abgar is said to have had a nephew named Sanatruk, "king of another province in Great Armenia", who converted to Christianity when he was visited by Thaddeus, but later recanted and had Thaddeus murdered on the slopes of Mount Ararat.  

In Armenia, Thaddeus met with the Armenian King Abgar, who reigned in the city of Edessa from 1 B.C. to A.D. 37. King Abgar had contracted a grave illness during his travels to the East. Hearing of the miracles of healing by Jesus of Nazareth, he wrote a letter to Jerusalem inviting Christ to Edessa. A delegation headed by the king's emissary Anan delivered the letter to Jerusalem, where it was received by the Apostle Thomas.

The delegation may have been among the pagans the Apostle John wrote about in his account of Christ's entry into Jerusalem: "and there were certain pagans who had come to Jerusalem to worship him (John 12:20)."

King Abgar's letter received its response after the resurrection and Pentecost with the mission of Thaddeus to Armenia.