If you aren't familiar with what an email feedback loop is, or why you would want to set one up, please read Email Feedback Loops first, then come back here.
Most email feedback loops work by sending you a notification each time a recipient marks an email that your registered IP address or DKIM key sent as spam. These notifications are referred to as spam complaints.
Spam complaints are usually, though not always sent in Abuse Reporting Format (ARF) with the original message and its headers attached. Depending on the ISP's policies, some details, such as the subscriber's email address may be redacted from the complaint.
How you process these complaints depends largely on what your relationship is to the sender.
Regardless of whether or not you're the sender of the email, you'll probably want to use spam complaints to monitor for issues. For example:
If you were the sender of the email being complained about, then I recommend treating spam complaints the same way as you would unsubscribe requests. Stop sending to the complaining subscriber promptly.
Both low tech and high tech approaches can be taken for deactivating subscribers. For example, you could: