The new Design editor is more mobile responsive than ever.
There’s a mobile view within the editor itself. You’ll be able to see how your campaign looks like on a mobile device, so you won’t need to send any campaigns to yourself to check how it looks on mobile.
As you can see elements that are next to each other will line up one after another in the mobile view. That is done in order to maintain a good quality of your images and retain the best view possible on the mobile device.
However, there are some sections that look awful placed this way. In that case, you can simply choose not to show particular sections on mobile devices.