421 4.7.28 / 550 5.7.28: Gmail Detected an Unusual Rate of Unsolicited Mail

A 421 4.7.28 from Gmail means Gmail is temporarily rate limiting you: it saw more mail, or more unwanted mail, than it expects from your IP, IP range, DKIM domain, SPF domain or a URL in your messages. It's a deferral, so your server will retry, but the retries keep failing until you slow down and fix whatever triggered it. If it continues, it can become 550 5.7.28, a hard block.

What the bounce says

Gmail documents several variants. The most common names your IP:

421-4.7.28 Gmail has detected an unusual rate of unsolicited email
421-4.7.28 originating from your IP address [203.0.113.7]. To protect our
421-4.7.28 users from spam, email sent from your IP address has been
421 4.7.28 temporarily rate limited.

The other variants swap in a different identity:

• "...originating from your IP Netblock [203.0.113.0/24]"
• "...originating from your DKIM domain [example.com]"
• "...originating from your SPF domain [example.com]"
• "...containing one of your URL domains."
• "Gmail has detected an unusual rate of email." (no "unsolicited," which suggests volume rather than complaints)
• "Gmail has detected this sender exceeded the quota for sending messages that have the same Message-ID:."

The permanent version reads "There is an unusual rate of unsolicited email originating from your IP address," and says the mail has been blocked rather than rate limited. Each version links to Gmail's sender guidelines.

What it means

Gmail tracks sending quotas per IP, per DKIM domain and per SPF domain, and it adjusts them based on your history and on how its users react to your mail. The variant tells you which quota you hit. That matters, because each one has a different fix:

Google's guidance notes that the IP quota is shared by every sender on that IP. On a shared ESP pool, another customer can use it up.

Common causes, most likely first

1. A volume spike. You sent much more than usual: a big announcement, a holiday push, a re-engagement send to old addresses, or your first send from a new IP or domain. Google specifically calls out doubling volume suddenly and sending in bursts.
2. Complaints and low engagement. Your spam rate in Postmaster Tools has climbed, or you're mailing people who never asked for it. This is what the word "unsolicited" is pointing at.
3. A shared IP or netblock. Another sender on the same infrastructure caused it. You'll see it on the IP or netblock variants while your domain-level metrics look fine.
4. A big change in infrastructure or format. New ESP, new IPs, new templates or a very different kind of message, sent at full volume with no ramp.
5. A listed or abused URL domain. A shortener or shared tracking domain in your links has a bad reputation of its own.

How to fix it

Google publishes specific steps for 4.7.28. Start with those:

1. Stop sending for at least 10 minutes. Continuing to hammer Gmail while rate limited doesn't help.
2. Work out which quota you hit. Read the variant. If it doesn't say, Google advises assuming DKIM, SPF and IP quotas are all affected.
3. Resume on a single connection. If it fails, wait another 10 minutes and try again.
4. Add connections back one at a time until you're at your normal level.

Then fix the underlying cause, or you'll be back:

1. Check Postmaster Tools. Look at your user-reported spam rate. If it's over 0.1%, and especially near 0.3%, complaints are part of the problem. See what the spam rate thresholds really mean.
2. Cut volume to engaged recipients. Pause sends to anyone who hasn't opened or clicked in a long time, and anyone acquired through a list purchase, co-registration or a pre-checked box. Send to recent engagers first, then widen slowly.
3. Ramp, don't jump. For a new IP, domain or ESP, start small and increase gradually. Google recommends ramping new formats and changed traffic separately.
4. If it's the IP or netblock variant on shared IPs, open a ticket with your ESP and include the bounce. They can see whether other customers on the pool are causing it.
5. If it's the URL variant, remove link shorteners, check every domain in the message, and use a branded tracking domain you control.
6. If it's the Message-ID variant, look for a retry loop or duplicate-send bug. That's an engineering problem, not a reputation one.

How to confirm it's fixed

Watch the share of Gmail-bound messages that defer with 4.7.28 in your MTA or ESP logs. It should fall as you reduce volume and rise again if you ramp too fast, which makes it a useful throttle signal. Over the following days, your Postmaster Tools spam rate should come down and stay below 0.1%. Spot-check inbox placement with a few real Gmail accounts. Senders that stop seeing deferrals but land in spam haven't recovered yet.

How to keep it from coming back

• Keep send rates consistent. Spread big sends over hours, not minutes.
• Use a sunset policy so unengaged recipients stop getting mail before they start reporting it.
• Use confirmed opt-in or at least a welcome series for new sign-ups, and never buy lists.
• Check Postmaster Tools weekly, not only during incidents.

When to get help

If 4.7.28 persists after you've cut volume and cleaned up targeting, or it has turned into 550 5.7.28, you're dealing with reputation repair rather than throttling, and a careful recovery plan matters. Get in touch.

FAQ

Is 421 4.7.28 a bounce or a delay?

It's a delay. A 4xx code is temporary, so your server queues the message and retries. If the retries keep failing until the queue expires (often after a few days, depending on your server), the message eventually bounces.

What's the difference between 4.7.28 and 5.7.1 "likely unsolicited"?

4.7.28 is rate limiting: Gmail is slowing you down. 550 5.7.1 rejects specific messages outright as likely spam. Both point at reputation, but 5.7.1 is the more serious signal. Low-reputation deferrals under 421 4.7.0 are another relative.

Can I ask Google to lift the limit?

Google doesn't accept allowlist requests. Its Sender Contact Form is for senders who meet the guidelines and still see excessive spam classification, and senders with a spam rate above 0.3% aren't eligible for mitigation until it stays below 0.3% for 7 consecutive days. Fix the cause first.