My app does not mess with other apps (how would it? that would be a major security hole in Android).
In fact, I've got Gmail's native app enabled on some of my devices, alongside my app (for the same account and more), and K-9 and Android built-in Email on some others.
Not trying to question what you were seeing, just saying the app does not try to do that.
Oh, we've got a Samsung Tab where the built-in Email is also enabled and working alongside my app.