Gmail 421 4.7.28 Rate-Limit Error - How to Fix

Gmail's 421 4.7.28 error is a temporary IP rate-limit deferral: your sending IP is pushing more mail than its current reputation supports, so Gmail throttles you and asks you to retry later. The 4 at the start means temporary, not permanent, this is throttling, not a ban. It almost always hits a new or cold IP, a sudden volume spike, or an IP whose reputation dipped after complaints. The fix is to send slower, queue with proper retry, and warm the IP so Gmail raises your rate ceiling. Once your send rate matches your reputation, the mail goes through.

What 421 4.7.28 actually means

421 4.7.28 is Gmail's way of saying "you're sending too fast for who you are." The full response reads roughly: 421-4.7.28 Gmail has detected an unusual rate of unsolicited mail... To protect our users from spam, mail has been temporarily rate limited. Gmail assigns each sending IP a throughput ceiling based on its reputation and history, and exceeding that ceiling triggers the deferral.

Read the code itself:

• 421 = transient negative response. The connection is being closed, retry later.
• 4.7.28 = the specific reason: IP/domain reputation rate limiting.

Because it's a 4.x.x (temporary) code, the correct response is to retry with backoff, not to treat it as a rejection. A well-behaved mail queue handles this automatically. The danger is a misconfigured sender that retries aggressively, which makes Gmail tighten the limit further. This is different from 550 permanent rejections like 550 5.7.1 message blocked.

Why your IP is hitting the rate limit

You hit 421 4.7.28 when your send rate outruns your IP's reputation, and three situations cause that most often. Each points to a different fix.

A brand-new dedicated IP has zero history, so Gmail's ceiling for it is very low. Push 50,000 messages through it on day one and you'll see 421 4.7.28 almost immediately. On cold IPs we've provisioned, the practical day-one ceiling to Gmail often sits in the low hundreds of messages, not thousands, and the deferral shows up after only a few hundred sends, not when the batch is half done. The mail isn't lost, the queue just keeps retrying it, but a 50K batch can take many hours to drain at that rate, which is exactly why the warm-up exists. The same happens when a normally steady sender suddenly blasts a large batch, or when a complaint spike has quietly lowered the IP's standing. Check your complaint rate in Google Postmaster Tools to rule out a reputation problem.

Step-by-step fix for 421 4.7.28

Slow down, queue correctly, and warm the IP. The deferral clears as your send rate aligns with your reputation. Work these steps in order.

1. Confirm it's transient and let the queue retry. A 4.x.x code means retry. Make sure your SMTP queue uses exponential backoff (wait longer between each retry) instead of hammering Gmail. Aggressive retries make it worse. See email queue and retry.
2. Reduce your send rate. Lower messages-per-hour to Gmail. If you're bursting, spread the batch over a longer window.
3. Use connection pooling, not floods. Reuse a small number of connections rather than opening hundreds. Flooding also trips 421 4.7.0 too many connections, a related error covered in 421 4.7.0 fix.
4. Warm a new IP. If the IP is new, follow a warm-up schedule: start around 50-100 messages a day to Gmail and ramp over 4-6 weeks. The ceiling rises as clean history accumulates.
5. Fix reputation if complaints are high. If Postmaster shows complaints over 0.3 percent, clean your list and suppress dormant recipients before pushing volume again. See list cleaning.

Per-IP throughput limits and how they scale with reputation are detailed in how many emails per day per IP.

How to confirm the rate limit has cleared

Watch your mail log for the retry outcome. After a 421 4.7.28 deferral, your queue retries, and a successful retry logs a 250 acceptance from Gmail. A log line like this is the confirmation:

to=<[email protected]>, relay=gmail-smtp-in.l.google.com,
  delay=412, status=sent (250 2.0.0 OK ... - gsmtp)

If you instead see the 421 4.7.28 line repeating on every retry, you're still over your ceiling, so slow down further. Check your queue depth too. A healthy queue drains steadily after you reduce the rate. A queue that keeps growing means you're still feeding Gmail faster than it will accept. In Google Postmaster Tools, your IP reputation and the "rate limited" volume should trend down over the following days as the warm-up holds.

Prevent 421 4.7.28 from recurring

The deferral comes back whenever send rate outruns reputation again, so the prevention is about pacing and steady volume.

• Warm every new IP fully. Don't carry production volume on an IP younger than 4 to 6 weeks. Ramp from 50 to 100 a day.
• Cap messages per hour to Gmail. Set a per-provider rate limit in your sending software so a batch can't burst past your ceiling.
• Keep volume consistent. Sudden spikes after quiet periods re-trigger throttling. Smooth large sends across hours.
• Hold complaint rate under 0.3 percent. Gmail's bulk sender guidelines set this ceiling, and a complaint dip quietly lowers your throughput. Clean your list and suppress dormant recipients before big sends.
• Tune queue backoff once. Exponential retry intervals mean deferrals self-heal instead of hammering Gmail into tightening the limit.

How BulkEmailSetup helps

421 4.7.28 is almost always an unwarmed or under-reputation IP being pushed too hard. We provision dedicated IPs with a managed 4-6 week warm-up that builds the Gmail reputation needed to raise your rate ceiling, plus SPF, DKIM, DMARC, and PTR configured correctly and queue tuning that retries deferrals cleanly. We monitor reputation so a complaint dip doesn't quietly throttle you. Plans are on our pricing page.

Frequently asked questions