In this case, deliverability is MailChimp's job...not ReturnPath's. MailChimp says so on their site. They have feedback loops with ISPs to show you opens, bounces, spam complains etc.

The "Inbox Inspector" add-on shows screenshots of what your email looks like in a handful of email clients and runs what I'm guessing is a SpamAssassin check that returns a spamminess number. If this add-on is from ReturnPath, then the service they are providing isn't exactly "deliverability" once you've sent --- it's really a toolkit to inspect and QA your messages BEFORE you send them.