Hi Jürgen, thanks for contacting us here!
Webrotate 360 product viewer requires full browser support to function. Also, the email clients and sites e.g, thunderbird, gmail, etc do not allow loading external javascript so the newsletter will only work as long as you would redirect your visitors to your website for full 360 product viewing experience.
Another option to consider for newsletters specifically is to publish your demos into animated GIFs as they will work pretty much everywhere and can be easily embedded as newsletter images. We have recently released animated GIF publishing feature so you can use existing SpotEditor projects to publish your 360 product spins as supplemental animated GIF images using the same project settings:
http://www.webrotate360.com/create-animated-gif.html