What is “complaint feedback-based blocking”?

Imagine sending a campaign and watching your delivery rates fall off a cliff at one specific provider. No bounce message you recognize, no blocklist entry you can find. What's happening? It could be complaint feedback-based blocking, and it means enough of your recipients voted your emails spam that the mailbox provider decided to stop letting you in.

Here's how it actually works. When a recipient hits the spam button, that signal goes back to the mailbox provider. Gmail, Yahoo Mail, and Outlook all aggregate these signals across all senders, constantly. When your complaint rate crosses a threshold, they don't necessarily send you a polite warning. They just start rejecting your mail.

The thresholds that matter:

• Above 0.08-0.1% (roughly 1 complaint per 1,000 emails): You're on providers' radar. Expect some filtering to tighten.
• Above 0.3%: Serious territory. Deliverability starts dropping noticeably.
• Above 0.5%: Blocking becomes likely at major providers. Some will reject outright.

These aren't universal hard lines. Different providers weight complaints differently, and the volume you send matters too. A 0.3% rate on 100 emails is very different from 0.3% on 500,000. But as a working guide, 0.1% is where you act, not where you panic.

The connection to feedback loops (FBLs) is where a lot of senders miss something important. When you're registered for a provider's FBL, each complaint comes back to you as a notification, so you can suppress that address before they complain again. Ignoring FBL data doesn't make the complaints disappear. It just means the provider is counting them and you're not doing anything about it.

But you might see block messages like "rejected due to user complaints" or "too many spam reports from recipients." Or you might just notice a sudden drop in delivery to one specific provider while everything else looks fine. That second pattern is sneakier and easy to miss if you're only watching aggregate stats.

If you're already in this situation, the fastest path forward is to honor FBL unsubscribes immediately, review where those subscribers came from, and look honestly at whether your content matches what people signed up to receive. Sometimes complaint spikes trace back to a single campaign, an aggressive re-engagement attempt, or a list segment that never should have been mailed. Identifying the source matters more than just waiting for complaints to drop on their own.

If your list has grown stale or you're unsure who on it is still genuinely engaged, that's a good time to look at your list health before sending more. Our RME Clean service can help you spot and suppress addresses that are quietly dragging your reputation down. Or if this is actively breaking right now, our SOS hotline is free and we actually pick up.