Hi,
The notifications are enabled by default, but only when there are 1) changes on the server and 2) there are new unread messages received during the most recent mail check.
Also, the app is defaults to not post notifications when you check mail manually, in the app (since the unread message counts are right there).
All of these can be changed in the app's settings, scroll down to the Notifications section. If you switch the app settings into Extended mode first (top item), there will be more settings items there.
Also worth checking: long press the account, check Special Settings to see if there is an override there (i.e. no notifications for a specific account).