Steps to replicate :

1. Create a new user in Confluence
2. Add an event to the calendar and ensure that you add the newly created user to the "Who" text field
3. Set the "When" to 10 minutes from the current time
4. Reminder set to 5 minutes before
5. Disable the newly created user immediately after that
6. Wait for about 5 minutes and the newly created user will receive a mail notification

Workaround
Change the user email address to another email address via the admin panel