This letter is a conformation of my receipt and acceptance of your resignation which you submitted on [some date]. While I would have preferred that we continue our fruitful cooperation for a longer period of time, I find your reasons to be legitimate enough and consequently understand your decision.
I appreciate the early notice and would like to thank you for years of commitment and dedication. Finding a suitable replacement will not be an easy task. Please continue to perform at your peak and help us in transferring your responsibilities as needed. Good luck in your next challenge.