421 4.7.0: Gmail is temporarily deferring mail from your IP

What the deferral means

A 421 4.7.0 response from Gmail reads like this:

421-4.7.0 [203.0.113.9] Our system has detected an unusual rate of
unsolicited mail originating from your IP address. To protect our
users from spam, mail sent from your IP address has been temporarily
rate limited.

The 4 at the front is the operative part. This is a deferral, not a rejection: Gmail told your server “not right now,” your server keeps the message in its queue, and it retries automatically over the following hours and days. Much of the deferred mail arrives on a later attempt. Nothing has bounced yet; what you are seeing in your logs is Gmail slowing your IP down while its unsolicited-mail signals, a volume spike or a rising complaint rate, sort themselves out.

Why Gmail throttles an IP

The trigger is the same family of signals behind the permanent 550 5.7.28 block, at lower intensity: the IP is sending more than its history supports, or recipients are marking the mail as spam. A big campaign to a stale list, a new IP ramping too quickly, or a compromised account leaking spam alongside your real mail all produce it. Gmail responds proportionally, and the deferral is the proportional response: enough friction to protect users, with the road back left open.

How it clears, and what not to do

• Let the retries work. Your server is already doing the right thing. Do not manually resend deferred messages; that doubles the volume on an IP Gmail already considers too loud.
• Pace the volume. Spread large sends over hours or days instead of one burst, and hold new campaigns until the throttle clears. The fix is pacing, not a delisting form; no form exists for this code.
• Clean the list. Complaints are the other half of the signal. Cut recipients who never engage, honor unsubscribes instantly, and stop mailing anyone who did not ask for it.
• Do not blast harder or rotate IPs. Pushing more volume through the throttle, or hopping to a fresh IP with zero history, are both spam-pattern moves that make the verdict worse instead of better.
• Watch it persist. A 421 4.7.0 that continues for days while you keep sending is on the escalation path to the permanent 550 5.7.28 block. Treat the deferral as the warning it is; check your complaint rate and IP reputation in Google Postmaster Tools rather than waiting for the hard bounce.

Frequently asked

What does 421 4.7.0 mean at Gmail?

Gmail saw unsolicited-mail signals from your IP, a volume spike or rising complaints, and is temporarily rate limiting it rather than rejecting outright. The 4.x code means it is a deferral: your mail server keeps the message queued and retries automatically, and much of the deferred mail is delivered on a later attempt.

Do I need to resend messages deferred with 421 4.7.0?

No. A 4.x deferral tells your sending server to retry, and every normal mail server does so automatically for days before giving up. Manually resending on top of the automatic retries only adds volume to an IP Gmail already thinks is sending too much.

How long does the 421 4.7.0 rate limit last?

Gmail does not publish a duration. The throttle follows the signals that caused it: when the volume spike subsides and complaint rates settle, deferred mail starts flowing again, typically without any action beyond pacing. If the behavior continues, the deferral can escalate to the permanent 550 5.7.28 block instead of clearing.

Should I switch to a different IP to get around it?

No. Rotating IPs is the signature move of a spammer and it resets you to zero reputation, which makes the throttling worse. Keep the same IP, cut the volume, fix the list or the compromised source, and let the reputation recover where your history already lives.