A feedback loop is a data feed which forwards complaints originating from their users back to the sender of the original message.
This complaint data is then processed by the sender (You) so they can remove and/or permanently unsubscribe the user from the list(s) in question. This helps to avoid future complaints.
People who generate a lot of complaints can start causing problems with delivery for themselves and anyone else on the same IP address.
On average, a complaint rate over 0.25% (1 in 400) at any given ISP for a given mailing is considered a problem.
How we calculate the complaint rate:
total number of complaints / total number of email sent to the FBL domains
It is not divided by the total number of emails sent as this would not give the most accurate representation of your true complaint rate at each ISP.