What is an FBL (Feedback Loop)?
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When someone marks your email as spam in their inbox, what happens next depends on whether the mailbox provider runs a Feedback Loop (FBL). If they do, they send a notification back to the sender: "Someone complained about this message." That notification is the feedback loop.
FBLs work using a format called ARF (Abuse Reporting Format), which is a standardized email report containing information about the complaint. Your ESP receives these reports and uses them to automatically add the complaining address to your suppression list, so they won't receive future emails. This is the right response. Continuing to mail someone who hit the spam button is how complaint rates climb.
Not every mailbox provider runs an FBL. Outlook does, and it's one of the more well-known ones. To receive those reports, senders or their ESPs need to register for the Outlook FBL program. Most major ESPs like Mailchimp, SendGrid, and Postmark handle FBL registration automatically for domains sending through their platform. If you're on a major shared ESP, your FBL is probably handled for you already.
Gmail doesn't offer a traditional FBL. It doesn't send individual complaint reports to senders. Instead, it provides aggregate complaint rate data through Google Postmaster Tools. Understanding that Gmail handles this differently is important for setting up your complaint rate monitoring correctly.
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