To resubscribe a Manager to their email notifications, navigate to Menu > Manage > Notification Config.
Then on this page, under the Email Test Facility, select the Manager and click the Test button, this will send an email to the Managers email, if they are unsubscribed, a prompt to resubscribe the manager will appear.
Managers can also resubscribe to the mailing list by accessing a previous management email and clicking "unsubscribe"