...
Users who are implicitly associated with work items (e.g. task assignees) can toggle their email notifications as needed from "My Settings" under the user avatar. For exact steps, refer here. On the other hand, when users are explicitly added as followers either by themselves (i.e. clicking on the 'Follow this item') or by another user, emails are automatically sent to them on updates to work items. There are no separate email preferences for explicitly added followers.
...