With this tool, you can see detailed information about the messages that Gmail receives from your domain. This could include emails from your regular mailboxes, marketing emails, transactional (such as those sent from Postmark) and more. For instance, you can see domain and IP reputation, spam rates, encryption usage, and email authentication success/failure rates.
If this tool shows a poor reputation or high spam rates, there are clear issues that you need to fix. In addition, you should support DKIM, SPF and encryption. Postmaster Tools will point out the items you are doing right (or wrong) in an effort to help you become a better sender.
It’s nice to see more tools like this being launched. It places more control into the domain owner’s hands instead of email delivery being a black box from the email service providers.