When a subscriber marks an email as spam, ESPs (Email Service Providers) take several actions primarily focused on protecting both the recipient and the sender's reputation. The core mechanism involves using feedback loops (FBLs) with ISPs to receive notifications of spam complaints. Upon receiving a complaint, ESPs typically unsubscribe the recipient from future mailings or add them to a suppression list, which prevents any further emails from being sent to that address. This action is documented in the customer's record. Legitimate ESPs always stop mailing users who mark emails as spam, although the specific method (special flag, unsubscribe, or suppression list) may vary. Ignoring spam complaints is not an option. Maintaining good list hygiene and actively managing complaints are crucial for improving email deliverability and preventing negative impacts on sender scores. Proper data management by brands is also important. Some ESPs provide tools for senders to monitor complaint rates and recommend a re-engagement campaign.