One of the panels I attended at Oasis 25 (literary SF con) last month was called Best Rejection Letters. The writers/panelists gave a variety of helpful hints to writers. Their advice included the reminder that even the best authors have been rejected. “Keep trying.” One writer on the panel received many rejections and cursed the editors. Now that he’s an editor and sends them out, he tries to remember how it felt. Another writer pointed out that rejections are just part of the process. Encouraging rejections are good. Do not respond to a rejection letter, unless it is an encouraging one. Then you can ask–“Would you like me to send you something else?” And if you can’t follow directions (the guidelines), it won’t be read! Watch your attitude. Some editors will not work with a problem-maker.
[I’ve received many rejection letters and stopped saving them long ago except for some of the encouraging ones.]