Mobile notifications can be enabled (or disabled) in your Settings. Please follow the instructions below to do so. If notifications are enabled, you can choose to be notified of messages in your primary inbox only (recommended), all mail, or a split inbox. Additionally, notifications can be set on a per account basis to all connected accounts in the app.
With the app open, tap on your OnMail avatar
On your Notifications page, you can toggle your notifications ON or OFF.