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Bounce error codes · Gmail and Google Workspace

Gmail 421 4.7.28: unusual rate of email

Gmail slowed down or blocked your mail because it saw more of it, or more unwanted mail, than it expects from your IP address, domain or links.

The exact error texts

  • Your IP address

    421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address ip-address. To protect our users from spam, email sent from your IP address has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.

  • Your DKIM domain

    421 4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.

  • Your SPF domain

    421 4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your SPF domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.

  • A link in your messages

    421 4.7.28 Gmail has detected an unusual rate of unsolicited email containing one of your URL domains. To protect our users from spam, email with the URL has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.

  • Repeated Message-ID

    421 4.7.28 Gmail has detected this sender exceeded the quota for sending messages that have the same Message-ID:. To best protect our users, the message has been temporarily rejected. For more information, go to Why has Gmail blocked my messages?.

  • Permanent version

    550 5.7.28 There is an unusual rate of unsolicited email originating from your IP address. To protect our users from spam, email sent from your IP address has been blocked. To review our bulk email senders guidelines, go to Email sender guidelines.

Quoted from Google Workspace — Gmail SMTP errors and codes

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What 4.7.28 means

Gmail tracks volume, recipient feedback and limits per domain and per IP address. A 421 4.7.28 means one of those quotas was exceeded. Your server should retry later on its own. 550 5.7.28 is the permanent version for an IP: those messages won't be retried.

The bounce text tells you which quota it hit. That matters, because Google says DKIM and SPF quotas belong to your domain, while an IP's quota is shared by every sender on it. On a shared IP, another customer's sending can rate-limit you.

Google's back-off procedure

Google's sender guidelines give an exact procedure for this error:

  1. Don't send email for at least 10 minutes.
  2. Identify the cause. If the error doesn't say whether you exceeded your DKIM, SPF or IP quota, assume all three are affected.
  3. After 10 minutes, send from a single connection.
  4. If that connection fails, wait another 10 minutes.
  5. If it succeeds, increase the number of connections one at a time.

Google's procedure starts with a pause. If your sending service handles retries, let it back off instead of pushing the same batch again by hand.

What usually triggers it

  • A sudden spike. Google warns that even doubling your usual volume suddenly can cause rate limiting. New IPs and domains need a slow start with engaged recipients.
  • Unwanted mail. The “unsolicited” variants mean recipients mark the mail as spam. Google requires a spam rate below 0.3% in Postmaster Tools.
  • Bursts. Google recommends sending at a consistent rate, not all at once.
  • A link with a bad reputation. The URL variant limits every message that contains that domain, including link shorteners or tracking domains shared with other senders.
  • The same Message-ID on many messages. A broken template or script that reuses one Message-ID. Every message needs its own.

Preventing it

  • Ramp up new volume gradually. Daily sending lets you increase faster than weekly. Ramp a changed email format or new infrastructure separately.
  • Send only to people who opted in, confirm new addresses, and drop recipients who never open.
  • Make unsubscribing easy: bulk senders need one-click unsubscribe (see Gmail's requirements).
  • Authenticate everything so the right reputation is used: SPF, DKIM with your own domain, DMARC. The check above shows what's missing.

Questions

Will my messages still be delivered?

421 is temporary, so your server keeps retrying. Messages are delivered if the rate drops back within the limit before your server gives up. 550 5.7.28 messages are not retried.

I use a sending service and didn't change anything. Why?

If the bounce names an IP address, the quota is shared by every sender on that IP. Ask your provider whether the IP is shared and whether they can move you. If it names your DKIM or SPF domain, it's about your own mail.

Is 4.7.28 the same as being on a blocklist?

No. It's Gmail's own rate limit, not a public DNS blocklist. Our blacklist checker shows public lists if you want to rule those out.

How do I see my spam rate?

In Google Postmaster Tools, after verifying your domain. Google requires it to stay below 0.3%.

Sources

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