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A few mailbox providers notify senders when individual recipients mark their emails as spam in their mailbox. This service is called the Feedback Loop (FBL). These reports allow senders to estimate how frequently recipients mark their emails as spam. Senders should remove senders in these reports from their list. FBLs are not IP- or domain-based. They are free to use when available. A few notable mailbox providers have an inbuilt FBL, including Gmail.
Every email marketer or sender should have a suppression or exclusion list. This is the list where you add people who have submitted unsubscription requests or spam complaints, or people whose email addresses return a hard bounce meaning their addresses are not valid anymore. We mentioned hard bounce here because soft bounces are only temporary – the address can be retried for a specific period after the first rejection.
Sending emails to your suppression list members repeatedly will damage your sending reputation. ISPs frown against this and automatically limit the delivery of your emails into the inboxes on their network. In the end, every email from you will be refused, as your sending reputation will continue to drop.
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