Dear [first name],
I am extremely grateful for the opportunity to read your submission, but I am unable to make an acceptance this time.
It’s always difficult to send back a rejection to a writer. I regret that we cannot offer a feedback for every submission we receive.
I wish you the best of luck and hope that you will keep reading our magazine in order to get a feel for the work we publish and that you’ll consider sending another submission in the future.
Kind regards, Diana