Feedback Loop and ISP Complaints
A Feedback Loop (FBL) is a system in which receivers (mainly ISPs) send feedback to the sender for recipients that complain or mark an email as spam. All major FBLs are configured through the Higher Logic system to automatically unsubscribe recipients that submit these complaints; they are classified as an ISP Complaint in the subscriber record. Keep in mind that ISPs do not share all complaint data with senders, but they will typically share enough to provide some insight into how a sender is being perceived by their audience.
Complaints are detrimental to your sender reputation, so it is best to do everything you can to avoid them. If you are seeing a lot of complaints, check the source of your lists, purge invalid addresses and un-engaged subscribers from existing lists, and make sure you have a clear unsubscribe mechanism in place.