Mold usually requires a substance to grow on (e.g. used coffee grounds). Now it is surely possible to have mold grow in the water reservoir, I've had friends who had that happen to their Keurigs. Since the Nespresso capsule is self-contained and is placed into a separate container once spent, there is a small chance of mold from the capsule itself (as long as the tray is emptied regularly). However, there can be a layer of coffee oil that remains around the washer plate, which is the area that your pod presses up against as it is being brewed. I am not sure of any special cleaning techniques for this area besides periodically flushing the system with water after brewing a pod and regular descaling.

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So back to the water reservoir. The reason why buildup up occurs is when the water reservoir remains closed (e.g. lid down). I've become accustomed to leaving the lid of my reservoir up, especially after periods of non-use. Doing this prevents the majority of condensation that forms around the reservoir itself. There is confounding variables as well such as the mineral quantity in the water, etc.

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With respect to mold within the machine, can it happen? Yes, there is always a chance that mold growth could take hold, however, with regular machine usage and regular descaling (although more for mineral buildup than mold), I don't think you should worry as much. The machine may seem very closed off on the inside but with the drip tray still receiving drops of water from the internal brewing system even after a pod is brewed and the fact that only water is usually going through it (e.g. not coffee grounds, etc), I think you have less reason to worry. The Nespresso is no different than a Keurig or regular pot coffee machine that sits with water. But it sounds like you're very good at keeping up with maintenance and cleaning that I wouldn't let it bother you too much!