The best lesson I have learned after dealing with lots of frustrated users is that their initial contact is more like how you'd interact with an inanimate object than a human. If someone yells at me personally, it seems a bit unreasonable... but if someone gets frustrated using my product and "virtually" throws it on the ground and stomps on it, that is a feeling I can understand. Once they have that off their chest, and I respond in a reasonable way, the follow-up conversation is always more respectful and humane. Just need to figure out how to read that initial communication and understand that they aren't mad at you, they are just frustrated with your product. Also, don't take it so personally that you miss out on the valuable bug report (or ux critique) that is so often embedded in these messages.
