Many email clients render and display HTML in a different way, this can lead to your beautiful newsletters being unreadable for certain recipients. We've made the preview tab of our campaign builder as accurate as possible but we always recommend testing your newsletter across multiple devices and mail clients.
If you've got some mobile-specific styles in your campaign or template, the mobile preview is a quick and easy way to see how those styles affect the campaign layout and display. However, the mobile preview isn't designed to show a 100% accurate representation for a specific device or specific mobile operating system.
You can see how media queries are applied to the campaign in landscape and portrait modes to get an idea of how your text, images and layout may appear on a mobile device. However, since there are a lot of different web browsers, email clients and mobile platforms, things may render a bit differently on the actual device compared to what you see in the Popup Preview. For more accurate previews, the best way is to send a test to a mobile email client.