While most of the rejections I ever received were of the form letter variety (“not at this time” or “does not fit our current needs”), I did get two that encouraged me to keep trying, perhaps for longer than was good for me.
Rejection #1
The first rejection was for a story that I had written and revised in my first writing workshop while I was pursuing a BA. It included an encouraging note with suggested changes that would improve the story. Not only did this rejection encourage me to submit and submit, it encouraged me to resubmit this particular story over and over again.