How do Gmail and Outlook report complaints?
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The two biggest mailbox providers in most commercial audiences handle complaint reporting in completely different ways, and understanding the difference changes how you monitor your sending reputation.
Outlook (including Hotmail and Live addresses) supports a traditional Feedback Loop (FBL). When a recipient marks your email as spam, Outlook sends you a report through the FBL in a format called ARF (Abuse Reporting Format). Your ESP receives these reports and should automatically suppress the complaining address. To receive them, you (or your ESP) need to be registered with the Outlook FBL program. Most major ESPs handle this automatically on your behalf.
Gmail works differently. It doesn't send individual complaint reports to senders, which trips up a lot of people. Instead, it shows complaint rate data in aggregate through Google Postmaster Tools. You can see your domain's spam rate over time, but you won't get a list of which addresses complained or the ability to automatically suppress based on that signal. The data is also only available if you're sending enough volume to Gmail to register (typically a few hundred emails per day minimum).
The practical implication: for Outlook, your ESP handles complaint suppression through the FBL automatically. For Gmail, you're responsible for monitoring your complaint rate through Postmaster Tools yourself. If you're not checking Postmaster Tools regularly, you might not realize your Gmail complaint rate has been climbing until your inbox placement drops. Set a reminder to review it monthly at minimum. See how to use Google Postmaster Tools for a setup guide.
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