Mail notifications continue to appear even when mail app is closed When using a variety of email clients (Newton, Nylas), MacOS will still show notifications for incoming mail when the client has closed. Is there a way around this?
Notifications for mail are designed to arrive whether the main app runs or not.

You can disable notifications for mail entirely, though - Go to System Preferences > Notifications and make sure the apps have their Notification settings turned off. In the example below, the DaisyDisk alert style has been set to None:



You can also uncheck all the checkboxes for more granular control of when you see (and hear) notifications.