The Postmaster Only Dies Once
Note: You probably won't get to finish this quest when you first run into it. Since the last NPC turn-in is in Pinnacle, which is unlocked later on in a story mission.
During your journey, you may come across this poor wounded Postman.
Speak to him and tell him that you will continue his final task for him.
The first on the list is really close to where he was. He was probably going to deliver it but he got ambushed.
Speak to the hermit to give him his letter.
The next person will be near the heart of the Farmlands.
Give Emlyn's letter to complete this part. (Note a book is behind her and triggers another side-quest "The Dragon Hunter's Journal")
The next one is located in the City of Light.
Give Faryn's letter to her to complete this task.
The last one requires access to Pinnacle and it is unlocked later in a story mission. So you can only continue this quest when you do have access to Pinnacle.
Give the last letter to Matilda to complete the side-quest.