Here's an old story I've learned a lot from. Once I had lunch with an ex co-worker of mine and the restaurant turned out to be an awful place - they brought us the wrong order, the waiter was impolite, the food was rubbish, the dishes were dirty etc.

I was going to ask for the manager, ask if there was a way to leave negative feedback - some book of complaints maybe, but my friend has suddenly stopped me.

"Don't waste your time optimizing someone else's business" he said. "If you want to punish them - not leaving any feedback would be best".

People feel the need to fulfill their anger, so sometimes filling a complaint seem to be a good way of "punishing" a company... Use that anger for your own good and offer them a way to "punish" you - a feedback form on your app's error page with a "what else we can improve" field would be nice. Cause the most negative feedback is no feedback at all. That's a real punishment.

So every time a customer sends you an angry email, remember - that's awesome. No email - would have been worse.