In the marketing module, you can filter by “has received a service in the last X weeks” and “has an appointment in a certain period”, but not by “has no appointment in the agenda”. For courses and cures (laser, multiple sessions with a fixed interval), that's exactly the point: if you haven't rebooked after their last session, we want to be able to write to you in a targeted manner. Now we are building the list of “service received in the last 16 weeks” and then manually check each customer sheet for a future appointment.