A return to sender spell is meant to return the negative energy or spell back to the person. The Karma ( should you believe in it ) remains the same.
This is where it gets tricky. A return to sender spell many people see as "good" magick because it is removing a spell placed on you by another. But it is also "bad" as you are taking a negative spell and returning it to another, thus placing a negative spell on someone.
I would double check with a seasoned Wiccan ( a real one ) as to the ramifications...but under the terms of Wicca and the threefold law I would imagine a return to sender spell to be classified as "black" magick as the theory of Karma is that the negative will be sent back to the original person with no actions needed by another magick practitoner.
**Please note I use the terms good, bad, and black loosely.