Starting on Monday December 14th, Google experienced a series of platform outages which impacted their Gmail service, for both GSuite and regular Gmail recipients worldwide. Google erroneously generated hard bounces indicating that legitimate mailboxes did not exist. The Google issues are now resolved. More information can be found here: https://www.google.com/appsstatus#hl=en&v=status
In Iterable, a hard bounce will automatically unsubscribe the recipient from that channel, and customers have seen unusual spikes in bounce rates and unsubscribes as a result of the issue. We are actively working on remediation efforts to roll back those unsubscribes. We are also working with our MTA partners who are going through a similar effort. Due to the scale of the impact, complete resolution could take some time. Once we have more information, we will provide a further update.