Generally speaking demanding data form your users before they get to experience/use your app is bad design. Its understandable to a degree for twitter ,tumblr, facebook, etc, because their services transcend mobile apps, and include a website that your account makes uniquely yours.
If all you are is an app, and the first thing I need to do is give you my email, to use the app (or register an account), I'm uninstalling it.
Sure, the idea is to try some (re)engagement hacks in the case they did not complete the "signup process".
Just so you know our app is an email application http://mailcloud.com/ that require people to add their email account.
If all you are is an app, and the first thing I need to do is give you my email, to use the app (or register an account), I'm uninstalling it.
:.:.:
Dare I ask why you want their email?