Looks more like working around deficiency in navigation, than giving more control.
Rethink your UX, put together prototypes, user-test, and repeat. Leave configuration option as last resort. That's the approach for app that wants to "Simplify my life" and "Make me amazing".
There are hundreds of design decisions that go into any product. Designers can't give each and every one a configuration setting, and can't know before launch which decisions will be the wrong ones.
If the back button did it's job properly then the feature would just work; you wouldn't need to manually configure it to match your mental model.
While I understand that users can be easily overwhelmed with a plethora of options, this one makes sense.. especially for an email app.