Is it possible to use the Mailgun list of "Bounces and Failures" to move unused accounts offline, or out?
Using Jamroom
Thinking about this some more it's not quite as straight forward as it may seem. Imagine that an email is sent to a user account, and it bounces due to a temporary issue (mailbox full, etc.). The system them "tags" that account as being undeliverable. Due to the way Mailgun works, it will not try to redeliver to that address. So you then come along and see that it is undeliverable and delete the account, even though the situation was temporary.
So my recommendation here would be to NOT look at email bounces, but rather look at last login - if your goal is to prune inactivate accounts, their bounce state isn't really going to do that for you. We have quite a number of users (hosted users even!) who have had their email bouncing for months and yet they are still active.