smtp4dev is a useful utility when you need to develop functionality that sends an email. In Windows, the program sits in your taskbar and listens to for emails. As you develop, it allows you to send the email directly to your computer.
When it receives an email, it displays it like an email client but it also allows you to look at header information. It can also open the email in your email client to see what it looks like there.
You can download it from the releases page on GitHub.