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421 4.7.28 error: Gmail rate limiting your mail

Gmail's 421 4.7.28 means it's temporarily rate-limiting you: it saw an unusual rate of mail from your IP or domain. Slow down; it's an early warning.

Sender problemYou are the problem: your domain, IP, authentication or content. Fix that before sending more.

What 421 4.7.28 means

Gmail421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address.

Other versions name your IP range, your DKIM or SPF domain, or a domain in a link in the message. One version means too many messages carried the same Message-ID.

List problem or sender problem?

Sender problem. It's about the volume, or the rate of unwanted mail, coming from your IP or domain. It's a temporary failure: the sending server retries on its own, but repeated deferrals are a warning.

How to fix 421 4.7.28

  1. Cut volume from that IP or domain and spread sends across the day.
  2. Review Google's sender guidelines before going back to full volume.
  3. If the text names a link's domain, take that link out of the copy.
  4. If it mentions the same Message-ID, make sure every message gets a unique one.

In SendingOS, 421 4.7.28 shows on the conversation with its reason and code. If bounces like this pile up on one domain, a domain health rule can pause that domain's inboxes before the whole campaign suffers.

Not sure whether a spike in bounces is the list or the domain? Read High email bounce rate? Burned domain or bad list.

421 4.7.0550 5.7.1450 4.2.1

Sources: Google: Gmail SMTP errors and codes

Questions

Will Gmail deliver the message eventually?

Often, yes: 421 is temporary, so your server retries. But Gmail is telling you it doesn't like what it sees. Treat it as the signal to slow down before it turns into a permanent block.