A complaint feedback loop (FBL) is a service offered by participating mailbox providers, in which they provide you with copies of complaints generated when your subscribers report your email as spam.
How FBLs work
Mailbox providers send you a FBL complaint notification when a subscriber marks your email as spam. Upon receiving the FBL complaint notification from the mailbox provider, the complaining subscriber is suppressed or removed from your email list.
While there are some differences in the way mailbox providers administer their FBLs, they generally operate in the same way by sending a copy of the offending email with full email headers.
Why FBLs are important
A high number of subscriber complaints negatively impacts your sending reputation and may cause your email to be blocked or sent directly to the spam folder.
FBLs allow you to positively impact your sending reputation by demonstrating to mailbox providers that by suppressing or removing complaining subscribers, it means you only send email that subscribers want to receive.
FBLs can also act as a signal to help determine the effectiveness of your list acquisition and content strategies. If you see high complaint rates across your campaigns, you can use the information to help troubleshoot and make improvements to your email program.