If you add a new appointment or edit an existing one, the reminder function is available under Appointment details. You receive an e-mail at your GMX e-mail address at the reminder time you set.
For each appointment, you can create up to five reminders with their own times. One reminder by e-mail is set by default. You can deactivate it in the relevant appointment or in the organizer settings. Changed organizer settings will only be applied to new appointments.