A Feedback loop (FBL) is a system where receivers (mainly ISPs) send feedback to the sender for users that complain or mark an email as spam. All major FBLs are configured through our system to automatically unsubscribe recipients that submit these complaints (classified as an ISP Complaint in the subscriber record). Do keep in mind that ISPs do not share all complaint data with senders; they will typically share enough to provide a bit of insight regarding how a sender is being perceived by their audience.
You can view complaints for your sent mail by navigating to Mailings > Reports > Mailing Results and then by selecting the particular mailing report you want to view. Within that report, click the Key Metrics tab to see the number of complaints and drill down to see whom they came from.
Complaints are detrimental to your sender reputation, so it is best to do everything you can to avoid them from happening. If you are seeing a lot of complaints (or just want to prevent them), Informz recommends that you check the source of your lists, purge existing lists that may have invalid addresses or un-engaged subscribers, and make sure you have a clear unsubscribe mechanism in place.