From Rejection Wiki
Look, We're all writers too, so we know how it feels. We're sorry we couldn't use your work. Because of our huge volume of submissions, we ask that thou please wait until our next reading period (May–November) before submitting again.Thanks for trying us. 2018 Thank you for submitting xxxx to us. We're sorry to say that we are unable to find a place for it at River Styx. Please know that every submission we receive is read by a human being who considers it with care. We hope that you will keep us in mind in the future, and we wish you the best of luck with your work.
As above, but with hand-written note. These are common, but not universal.