ISP Feedback Loops/Complaint Feedback Loops (FBL) is a process which allows mailbox providers a method to forward user generated complaints back to the sender or Email Service Provider. In order to receive the FBLs, the senders or Email Service Providers must first subscribe/register with the organizations which offers it. It is common best practice to treat/honor the FBLs as a user requested unsubscribe.
Current list of ISPs and organizations offering Feedback Loops are listed- List URL and reference links to ISPs and Organizations supporting Feedback Loops
AOL
Bluetie/Excite
Comcast
Cox
Fastmail
Hotmail
Mail.ru
OpenSRS/Tucows
Rackspace (formerly Mailtrust)
RoadRunner/Time Warner Cable
Synacor
Terra
USA.NET
United Online/Juno/Netzero
Yahoo!
Zoho.com