No doubt the observation below is well known to customer support veterans but for new MISVs taking customer support emails for the first time it’s worth knowing, meaning I wish someone had told me 
Often (but not always) the first email I receive from a customer with a problem or question seems a little rude.
At first I was a little taken aback, offended even. But time and time again their second email, their response to my reply, is much more polite and friendly.
In fact, the less friendly the first email the more friendly the customer is once they realize that you, a real person, will respond and care about their problem or question.
Obviously the customer isn’t trying to be rude it’s just they expect to get the run around, they expect to not be considered important. Once you show that you do care life gets much easier.
So i guess the lesson is to always keep your cool, be polite and assume the best of that person on the other side of the interwebs, a cranky email probably isn’t an attack on you and your product, it’s just someone who wants your help.