You can add a shadow beneath elements in your newsletter, for example at the end of it. An example:

You can add the shadow as a full-length picture. First search (for example with Google) a picture of a shadow you'd like to use and save the image to your computer. Or use this one (click on it, and choose "Save image as"):

Then, click on the "+" sign in the bottom left corner of your template, and drag a full layout row to the location you wish to add the shadow to. Drag a picture element inside the layout row, and then upload the shadow picture.

You also need to remove the padding from the picture. Click on the picture settings icon in the upper left corner of the picture.

The image properties window will appear. From there, remove the checkmark from "Vertical padding".

If the shadow doesn't look as wide as it should, adjust it manually from the same image properties window. Set the image width to be 600px, leave the height empty (read also Changing the size of an image).