This list was originally present the wiki for this mailserver repository wiki page. I've created a gist with it as it is something that can be useful per se and I'd like to keep a copy of it.
A Feedback Loop (FBL) is a system by which an Internet Service Provider provides the sender a copy of the message that one of their subscribers has reported as spam — usually by hitting a “report spam” button in that ISP’s mail interface.
ISPs offer feedback loops to qualified senders, so the senders can identify subscribers who no longer want to receive similar mail and/or any problems originating from their networks, such as virus-infected spam-producing machines.
This page summarizes the information about FBL signup pages, whitelist pages, and postmaster pages of most popular ISPs around the world.
| ISP | FBL Signup | Whitelist Signup | Postmaster Page | ----- | -------------- | ----------------