This application allows you to create lists with emails that can be checked against a DNS and Gateway to see if they work or not. Although this process isn't used nowadays, you can still use it, but it's uncertain about the replies you'll get. To use it, simply create a list and assign its name, then right click on the right side and add your emails. Once the emails have been added, start the checking process. You can watch the results in the Server Reply column and that's it.