All users are eligible to receive email notifications from Workfront.
The rules of an individual object restrict whether the user is eligible to be associated with a notification, based on their access level, and subsequently if they should receive an email, based on their notification settings. For more information about configuring notification settings, see "Configuring Email Notifications."
For example, since external users, reviewers and requestors cannot be assigned to a task or an issue, they will not receive an email notification when they are assigned to a task or an issue.
For more information about enabling event notifications and who receives the emails, see "Enabling and Disabling Event Notifications."