Please be advised that you can make rounding only for containers and structures.
Let's see how we can add border radius to the container via editor tools:
1. So first of all, select any block and go one step higher to the container.
2. After that set the background image or background color (if necessary) and the rounding.
Please note, when you add an image block to the container, rounding won't work because the image will replace this container.
Now let's see how we can add border-radius to the structure via code editor:
1. Select the needed structure and click on the code editor icon.
2. Add border-radius: 20px 20px 20px 20px; border-collapse: separate; (change the value in pixels to the desired) to the code editor and it's done.
Please note, with manual changes in the code, we can not guarantee that this will work in all email services since some may not support this kind of rounding.
Please contact us if you need any help.