The Exchange server provides message notifications to Email Notification Service (ENS) by pushing the notifications.
After the Email Notification Service (ENS) fetches the device and user details from Workspace ONE, it subscribes these device users to the Exchange endpoint through the Exchange Web Service (EWS). To subscribe these device users, ENS uses a service account with an impersonation role assigned to it. This subscription creates a connection between the ENS and Exchange server.
For push notifications, there is no open connection created between the ENS and the Exchange server after ENS subscribes the users to the Exchange. For a new message, Exchange pushes the message event to ENS and ENS then fetches the mail details from the Exchange. Exchange provides near real-time notifications and therefore polling of the Exchange server is not required.