Sends the appointment's attendee an email with the appointment details and links to reschedule or cancel it, mirroring the confirmation email they received when the appointment was booked. The appointment itself is not touched - use this when the customer should complete the change themselves (e.g. a chatbot that verifies ownership out-of-band) instead of the caller cancelling or rescheduling directly. The email is written in the language the attendee booked in. Returns the address the mail went to. The site must be published, since the links point at its booking widget; an unpublished site returns 400. When no appointment with this UID exists the API returns 400 with error_code ResourceNotExist, and an appointment whose attendee has no email address (a phone-only booking) returns 400.