Dear Mr Editor
Thank you for sending me your rejection letter. I have reviewed the letter and although it shows excellent promise, I am very sorry to say that I am going to pass on it. It is tightly written and I think you did well to attempt expand upon a classic subject but it feels too rooted stylistically in the old chestnut of a form letter rejection. Although your writing is strong, this particular piece does not add anything new to the genre. I think avoiding the popular fall-back of a negative ending would improve this work tremendously. At its current length, the negativity feels a little thin. Tastes vary by market, of course, but I'm afraid it does not suit our editorial needs at this time.
Please note that this does not necessarily reflect on the quality of your work: I receive about 15 submissions for every editor's letter that I accept.
I hope that you can take some consolation in knowing that your letter made it to the final round. I appreciate your thinking of me and I would be very interested in seeing further submissions from you.
I wish you the best of luck in placing your letter!