Mail app crashing - IOS 9.3.4

Hello Vincius Wielewski!

I'd certainly want my Mail app acting properly on my iPhone. We can check a few things with you.

If you haven't, try a force close of the Mail app and then restart the iPhone:

Force an app to close on your iPhone, iPad, or iPod touch - Apple Support

Restart your iPhone, iPad, or iPod touch - Apple Support

Afterwards, if it's still closing out on you, try updating the iPhone as a new update just came out today, iOS 9.3.5:

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

Then depending on what was done by the hard reset and reset of iOS, make a backup of your iPhone and then restore it to setup as new. Then you can add your email accounts and try out the app before putting your backup on.

How to back up your iPhone, iPad, and iPod touch - Apple Support

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

Let us know how those go.

Regards.