It shouldn't really come as a surprise that a basic web element like rounded corners would find its way into html email designs. This basic design element serves to sedate a design and bring a bit of calmness to it. By simply rounding out the corners the sharp edges are eliminated and a subtly relaxing connection is made. Sure this isn't an over the top effect, but all the same, these rounded edges serve to soften the designs in a way that makes them feel far less forceful.