This is a form letter that can come from killing various different NPCs.
A courier will never deliver this letter after killing one of these characters, according the Creation Kit:
|Show: Excluded characters|
This letter is delivered by a courier after the Dragonborn kills certain people, and informs him/her that killing the person, helped the sender of the letter. If the sender and victim were foes, this letter will be delivered. If the sender and victim were neutral or better, Letter from -NPC- (Thanks) will be send instead.
You may or may not realize this, but by taking care of <NPC>, you have indirectly helped me. Don't worry about how I know it was you, I won't be informing the authorities. But I would like to reward you for you efforts. Please visit me at your earliest convenience.