From my experience, one thing that Really helps to reduce the amount of support emails, is writing a easy to use FAQ (+ maybe a Wiki with user content) and point to that inside your app, and app description in the store.
We have the FAQ and forum, they're linked in the app listing and in the readme in the app. I shudder to think what our support load would be like without it. It's really only like 1-3% of the total users who have a problem that results in an e-mail. If I get a question a lot I put it there, but thanks to all of the variation possible on Android (many different hardware configurations, OS configurations, custom ROMs, software load running in the background, problems with external things like controllers and headsets, extras like Google Drive accounts) it's inevitable that it'll be an unholy mess that results in a lot of problems. You can look at pretty much any app sold and you will always see 1 stars from people saying they can't get it working.
It's even worse for an emulator because it doesn't Just Work out of the box, you need games. And people will ask for ROMs no matter how much you put it in bold all over the place that you can't help them. And they'll get broken ROMs. They'll get ROMs that are modified in unknown ways. And there's a million ways to use the emulator - thousands of games that we can't possibly always be testing and will never get fully working. And even worse, tens of thousands of cheats that do all kinds of awful things to the game code making a nearly infinite number of test paths (and there's no real guarantee that the cheat ever really worked properly to begin with because you know the people who wrote them couldn't play through the game with all possible permutations of cheats). And believe you me, people take their stupid cheats seriously.
Then you get people who complain because they don't know how to play the game. You wouldn't believe the number of people who blame the emulator because start doesn't open the menu in Pokemon. And that's the thing, there are always going to be some people who send angry e-mails first instead of trying things out or looking for some kind of answer. A sensible person would, when finding a button doesn't do what they thought it should, stop and try other buttons. Not close the game and give the emulator a one star rating with an incoherent review. But you can't expect all people to be sensible. Most of the e-mails are actually fairly reasonable and polite, but we do get some petulant types. And we get people who are using the cracked version and don't understand that it intentionally induces bugs - you'd think people would generally not bother developers if they didn't pay for the app, just out of some sense of pride at least. A simple "Did you get this from the Play store?" usually results in never hearing from them again.
And no matter what you put in the FAQ
and the app listing people will still constantly ask about wifi support or 3DS support (or something even more absurd like if we'll add cheat codes to let you use Gen 6 Pokemon).
There's also trolls, like this guy e-mailing from "nintendodsjapan@yahoo.com" with the name "president Takashi" blasting their terms of service page at us (where they complain about emulators, among other things) in what I can only guess is a really lazy attempt to fake a cease & desist. There's the usual people who want us to pay them to review the app, translate it, promote it, or give us 5 star reviews (I'm not kidding, we're solicited by people who are asking for money in exchange for guaranteed 5-stars). And there's just the plain old untargeted spam.
Suffice it to say, a big e-mail load was always going to happen. I'm just glad it's gone down from the first few days. Early on it was several dozen refund requests and "where do I get the BIOS" a day. Then it was a lot of dealing with various Pokemon bugs. Adding the custom BIOS and improving Pokemon were definitely the most important things we did to make things more manageable, it's probably only around 10-20% what the e-mail load was. But the stupid October bug opened a fun new can of worms (some people just do NOT update the app until you tell them they have to). But we really do try to answer - I don't like going to sleep with more than a few recentish e-mails undealt with. I did get someone from the forums to help out too.
And when Pokemon X/Y come out I expect to be flooded again.