Confluence mail notifications for users without groups

Description:
The Subscribing+to+Email+Notifications+of+Updates+to+Confluence+Content documentation mentions:

You will only receive notifications for content that you have permission to view. Users that have been disabled by an administrator will not receive email notifications.

However, if the user has no groups, they will not be able to view any content but will still receive notifications from Team Calendars, if they are have created the calendar.

Steps to reproduce:

1. Configure a Confluence to use Mail Server
2. Install the Team Calendars plugin
3. Create a new user with a valid e-mail address
4. Log in with this user
5. Create a new calendar with this new user
6. Log out
7. Log in with the admin user
8. Add the calendar created by the new user
9. Create an event on this calendar
10. Check the new user's mail box for the mail notification about the new event
11. Now remove all groups from the new user
12. Go to the Team Calendars again and remove that event you've just created or create a new one
13. Check the new user's mail box for the mail notification about the previous action

You should not receive any notifications since this user does not have permission to view anything on Confluence any longer, but the notification is received anyway.

Workaround:

1. Go to the User Management section
2. Find and select the new user
3. Click on the Disable button