Dear First Last,
We want to thank you for giving us the chance to consider "[Title]." Unfortunately, we aren't able to accept this submission at this time. If you haven't already done so, we hope you'll take a look at some of our issues to get an idea of the kind of work we're looking for. As a small operation committed to keeping our issues limited in length and frequency, we are unable to publish much of the fine work we are offered, such as your own.
Thank you so much for thinking of us, and best of luck with your writing.