You can choose to lock access to AltaMail and its data using a setting. Open the app settings (View) - Application section, near the bottom. Tap on 'Application Password' and then you can choose when to lock the app screen. You can enter your own password or a pin code. Note: if you forget your password/pin then you will need to reinstall the app.
You can also choose to use the touch id fingerprint unlock option if your iPhone/iPad supports it.