Each email account has its own switch for new-mail notifications and its own sound. Change them on your phone or Mac, and they apply on every device where you use Newton.
Steps
Desktop
This is on Mac only.
Press ⌘, to open Settings.
Go to Accounts and select the account.
Under Notifications, switch New emails on or off.
Choose a Sound: Default, None, Hubble, Juno, Jupiter, Sputnik or Voyager. You'll hear each one as you pick it.
To change how banners look, click Manage alert style and sound.
iPhone, iPad & Android
Open the menu and tap the Settings gear at the bottom.
Tap Accounts, then the account.
Tap Notifications.
Switch New emails on or off.
Under Sound, pick Default, None, Hubble, Juno, Jupiter, Sputnik or Voyager. You'll hear each one as you pick it.
To change banners, vibration or the lock screen, tap Manage alert style and sound.
Good to know
Newton for Windows and Linux doesn't show these settings. An account you switch off on your phone or Mac stays quiet there too. To silence Newton completely on Windows or Linux, use the system notification settings.
Only new mail in the Inbox sends a notification. There are no notifications for other folders.
These settings don't change notifications for read receipts, mentions or Recap.
On a shared team mailbox, each person chooses their own setting.