Moosend has the capability to inform you when an email you have sent out has been opened. An email is considered as "Opened" when the images of the newsletter are downloaded to your recipient's device. Since using the Preview Pane to preview your campaign does not download the newsletter's images, this will not be tracked as a campaign open.
Make sure that you get more information on how Moosend tracks and counts opens. This way you'll know exactly how your statistics work and what you can do to improve their accuracy!