I am using Windows 10 Enterprise Version 1709 Build 16299.309. I use the Windows 10 Mail App in German. When opening the settings from the lower bottom left, a pane appears on the right, where you can chose between different options. Chosing the options for reading (Lesebereich) shows the following screen.
I think, using Auto_öffnen is very unusual in German and sounds like something for programmers. Therefore, I suggest to change it. A suggestion is: Automatisches Öffnen. If it's too long, simply Öffnen would be ok too, I think, as from the option below (Nächstes Element automatisch öffnen), it is clear what the options are for.