What is the difference between 4.7.0 and 4.7.28 at Gmail?

They are the same throttle applied to different identities. 4.7.0 historically named the IP; 4.7.28 names the DKIM domain, SPF domain or IP in the brackets so you can tell which reputation Gmail is limiting.

Can I avoid this by sending from a different domain?

Only if the new domain has its own history and the mail is genuinely different. Moving the same list to a fresh domain gets that domain throttled sooner, and Gmail links domains that share DKIM keys or IPs.

Will my transactional mail be delayed too?

If it shares the domain, yes. That is the main argument for separate subdomains per stream.

421 4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your DKIM domain

Like 421 4.7.0 but keyed to the domain rather than the IP: Gmail is deferring mail signed by, or claiming, your domain regardless of which server sends it. The brackets name the domain and sometimes "SPF domain" or "IP address" instead of "DKIM domain", depending on which identity Gmail attached the rate to.

What the server replies

421-4.7.28 [example.com 12] Our system has detected an unusual rate of 421-4.7.28 unsolicited mail originating from your DKIM domain. To protect our 421-4.7.28 users from spam, mail sent from your domain has been temporarily 421 4.7.28 rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines.

Sent by: Gmail
Type: TemporaryRetry helps
About: Rate limits and quotas
Enhanced code: 4.7.28

Who sends it

Gmail inbound servers after RCPT TO, when a domain's recent volume toward Gmail is unusual for its history. Senders who use a large shared platform see it with "IP address"; senders on their own domain reputation see "DKIM domain".

Why it happens

A large one-off send from a domain that normally sends a trickle.
A new domain or subdomain that started sending bulk mail without a warm-up.
A compromised account or form that is pumping out mail under your domain.
Spam complaints concentrated on one domain even though the IP is shared and otherwise healthy.

How to fix it

Confirm who is sending
If you did not just send a campaign, assume a compromise. Check your provider's outbound log and your DMARC aggregate reports for sources you do not recognise, and reset credentials for any account that is busy.
Throttle the domain, not just the IP
Spread the remaining send over the next day or two. If your platform lets you, cap the per-domain rate toward gmail.com.
Separate streams by subdomain
Send marketing from one subdomain (news.example.com) and transactional mail from another so a newsletter cannot throttle password resets. Each subdomain carries its own reputation.
Watch Postmaster Tools
The domain reputation panel shows when Gmail has relaxed. Resume normal volume only after it reads Medium or High.

Check it yourself

These free tools show the records and connections behind this code for your own domain or server.

Related codes

Sources

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