Whenever someone unsubscribes from receiving emails they'll have a chance to provide additional information to help you understand why they opted out of future messages. The survey they will be presented with gives 5 options which allows you to get a better idea of how your audience is responding to campaigns and what changes you can make to help prevent individuals from unsubscribing in the future.
The best part about the unsubscribe survey is that you can use it on a list to see the full details of what individuals responded to on a specific list. This can help you determine the health and deliverability of a specific list. You can access this feature by going to your Lists tab, navigating to the list you would like to see the details of, clicking on the More Options icon (...) in the action column, and selecting Unsubscribe Details.
After clicking on Unsubscribe Details, you'll be able to see a pie chart of the amount of reasons and a list of each unsubscribe in the list and the specific reason they unsubscribed.