A feedback loop (FBL) is a service offered by mailbox providers, in which they provide you with copies of complaints generated when your subscribers report your email as spam. Mailbox providers that provide feedback loops include AOL, Comcast, Microsoft, and Yahoo!
How FBLs work
Mailbox providers send you the message your subscriber complained about so you can remove the subscriber 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
FBLs are important because they allow you to identify where complaints are coming from and can help you determine why subscribers are marking your email messages as spam. With this information, you can manage your lists by removing subscribers who do not want to receive your email, and implement other best practices to control complaint rates and protect your sender reputation.