Viewing mail in a new pop-out window is handy for those who wish to check through messages without having the UI of the main inbox present and cluttering up the screen. Unfortunately, this feature isn't available in the built-in Mail app for Windows 10, but this is about to change.
Brad Sams over at Petri managed to get his hands on an unreleased build of the app, which sports this very feature. Thankfully, with Microsoft looking to implement said functionality into the Mail app, those who rely on it for email management will be able to view multiple messages at once in a more feature-rich experience.
For now, we'll have to hold out until Microsoft eventually rolls out the feature in an app update.