Typically to sign up the person or organisation applying for the feedback loop will need to prove that they are responsible for the mail being sent from the IP addresses or domains on which the feedback loop is being setup on so be prepared to fax or send documentation and to be able to receive email on your domains abuse address.

Once verified and the application is accepted by the ISP the ISP will then start emailing an email address defined within the application each time one of your emails is reported as spam by the subscriber by clicking the "this is spam"button.

The email sent to this address will be an ARF message, abuse report format. This is an email identifying itself as an ARF message with an attachment of the original message that was sent to the complainant. The receiving mail server should pass this to either a human helpdesk if your volume is low to be unsubscribed from the list or alterntively be passed to a processing application if you send large volumes of email.

This processing application will read the ARF message and the email attached within it, identify either through the to email address or the headers in the message which subscriber is complaining and unsubscribe them from the lists automatically.

So how does this help me?

Although it may seem like a bit of a waste unsubscribing users who complain once, in the long term it will pay dividends. By unsubscribing users who complain once you are reducing the likelihood of them complaining again, this means your overall complaint rate per IP or domain is kept low which is the key metric that ISP's use to choose whether or not to deliver your messages. By keeping your complaint rate low from your messages to ISP's which have feedback loops they are much more likely to allow your messages straight through to the inbox ensuring you continue to get your emails to the subscribers who actually want to receive them.

In addition, some ISP's, after you have established a good record on their feedback loop, will actually allow you to apply for a whitelist which allows you to skip some if not all of the spam filtering that they use. AOL for instance is one ISP which operates a whitelist scheme in addition to a feedback loop.

Several feedback loops are available, some from very large ISP's. See below for details and the website for each feedback loop.

Yahoo! - http://feedbackloop.yahoo.net/ - You must have implemented domain keys to take part.

AOL - http://postmaster.aol.com/fbl/index.html - IP based

Hotmail/MSN/Live - http://postmaster.live.com/Services.aspx#JMRPP - IP Based

Cox Communications - http://fbl.cox.net/

Bluetie - http://feedback.bluetie.com/

Comcast - http://feedback.comcast.net/

USA.net - http://fbl.usa.net/

If you use an ESP to deliver your campaigns on your behalf, they should already have these feedback loops in place. Ask them and if not see if these can be implemented on your behalf.