You can make Newton on your iPhone, iPad or Android phone ask for Face ID, Touch ID, your fingerprint or your device passcode before it shows your mail. The lock is set on each device separately.
Steps
Open the menu and tap the Settings gear at the bottom.
Tap General.
Under Security, switch on Require Face ID. The name matches your device, so it may say Require Touch ID, Require fingerprint or Require passcode instead.
Confirm with your face, fingerprint or passcode.
To give yourself a grace period, tap Require unlock after and choose Immediately, After 1 minute, After 5 minutes or After 15 minutes.
When you come back to Newton after that time, it asks you to unlock it. If you cancel, you see Newton is locked. Tap Unlock to try again.
Good to know
Your device needs a screen lock. Without one, Newton shows Set a screen lock first and can't turn the lock on.
If Face ID or your fingerprint doesn't work, you can unlock with your device passcode.
While the lock is on, Newton hides your mail when you switch apps, so it doesn't show in the app switcher.
The lock doesn't sync to your other devices, and there's no lock in Newton for Mac, Windows or Linux.