Mail Designer 365 allows you to add borders to shapes and images. To start, insert an image or shape (in this example, we are using an image from Unsplash) into an image area.
Click on the image or shape to bring up the border tool in the "Style" menu.
Adjust the width of your border by clicking the arrows to increase or decrease width. You can also manually type in a value and hit enter.
Use the color picker to choose a color for your border.
Finally, choose a border style from the drop down menu to complete the effect.
Here are some examples of different styles:
Search through the manual