A complaint occurs when a message is marked as spam or sent to the spam folder by a recipient using their web email client. The Internet Service Provider records it as a complaint. If the ESP

has a feedback loop in place with the ISP, they are notified when they receive complaints. Your complaint rate can be determined by the number of complaints that are registered divided by 

the total number of mails sent. The most accurate way to keep track of complaint rates, considering not all ISP's have feedback loops setup with your ESP, is take the number of complaints sent

to a certain IP or domain and divide it by the number of mails sent. Complaint rates can affect the deliverability of your messages if they remain at an elevated level for a period of time. Complaints need

to be taken seriously and if you are able to trace a complaint back to a specific recipient you should remove them from your mailing list right away, so they don't have an opportunity to complain about

your mails again.