I generally do not recommend third party list verification services. They compromise the integrity of the security of your subscriber data and rarely have a real look at the transactional data that’s necessary to validate if a subscriber is valid or not.
I would encourage you to read thru this other thread on the same topic. Some of my Unsubscribes show this: "This subscriber has been unsubscribed manually via the control panel." - #12 by tomkulzer
If you’re interested in having the highest quality subscriber data, I would recommend using confirmed opt-in.