Many widgets that work with WordPress will work with Postmatic. Passive widgets work best. Since the widget will be displayed in your user’s email client, you should keep things simple.
As a rule of thumb, widgets that offer user interaction might not work. For example, a gallery slider might not function in some email clients, but a list of related posts (which are just links) would have no trouble. The best advice is to keep it simple.
We have a curated list of widgets with compatibility information in our directory.