421 4.7.0 Rate Limited or Temporarily Deferred: What to Do

A reply that starts with 421 is the receiving server saying "not now". The message was not rejected; the sending server was told to come back later, and it will, on its own, for a day or two. Most 4.7.0 deferrals clear by themselves. The ones that do not clear share a small set of causes: a sudden jump in volume, mail that is not authenticated, an IP or domain with a complaint problem, or a list full of dead addresses. This page explains how to read the exact wording, when to wait, and what to change so the deferrals stop.

Quick answer

A 421 is temporary. If you sent one message to one person, do nothing for a few hours; the sending server retries automatically. If you see it on many messages, or it keeps returning for days, the receiver has rate limited your sending domain or IP. Make sure SPF, DKIM and DMARC all pass, cut the sending rate, remove addresses that bounced, and check the IP on the public blocklists. The limit lifts within a few days of clean sending.

The common wordings

• Gmail: "421-4.7.0 [IP] Our system has detected an unusual rate of unsolicited mail originating from your IP address ... has been temporarily rate limited." The IP or domain exceeded what Gmail expects from a sender with your reputation.
• Gmail: "421-4.7.0 Try again later, closing connection." Often a burst of connections or a sender with no history; usually clears the same day.
• Yahoo and AOL: "421 4.7.0 [TSS04] Messages from [IP] temporarily deferred due to unexpected volume or user complaints." Yahoo deferrals come from complaint rate more than anything else.
• Outlook.com: "421 RP-001 The mail server IP connecting to Outlook.com has exceeded rate limits imposed on this IP." A per-IP ceiling for senders without an established reputation.
• Any Postfix or Exim server: "421 4.7.0 ... too many connections" or "try again later". A connection-rate limit on the receiver, not a judgement on you.

Why it happens

• Volume jumped. A domain or IP that normally sends 50 messages a day suddenly sends 5,000. Receivers slow new volume down until they see how recipients react.
• authentication (proof that email really comes from your company) is missing. Unauthenticated mail gets the strictest limits. Since 2024 Gmail rejects it outright with 550 5.7.26, but partial setups (SPF only, DKIM for another domain) still land in the rate-limited bucket.
• Complaints or spam-trap hits. A few "report spam" clicks on a small list are enough for Yahoo to defer everything from the IP.
• Dead addresses. Sending to many addresses that bounce looks like a purchased list, and receivers defer the rest of the batch.
• Shared IP reputation. On a shared sending IP, someone else can trigger the limit. Hosted services watch for this; a VPS you run yourself does not.

What to do, in order

1. Read the full response in your delivery log. The sentence after the code names the cause (rate, authentication, complaints) and often includes a link to the receiver's sender guidelines.
2. Confirm authentication with Show original on a message that did arrive at a Gmail address: SPF (Sender Policy Framework — a list of servers allowed to send email as your domain)=pass, DKIM (a digital signature that proves your email was not tampered with)=pass and DMARC (a policy that tells providers what to do if someone fakes your domain)=pass, all for your own domain. Fix whichever is missing before anything else.
3. Pause bulk sends for the day. Do not retry by hand; the sending server already retries. Retrying from a script on top of that makes the rate problem worse.
4. Remove every address that hard-bounced in the last send, and anyone who has not opened in a year, before the next one.
5. Check the sending IP on the public blocklists and request delisting if it appears. Our blocklist guide walks through it.
6. Restart at a lower rate and grow it over two weeks. A warm-up schedule for a new domain applies just as well to a domain coming back from a deferral.

If you send through OquMail

Retries are handled for you: a deferred message shows as deferred in the delivery log and is retried on a growing schedule (5, 15, 30 and then 60 minutes apart) until the attempt limit is reached, after which it is recorded as failed. The platform limits also keep you well inside most receivers' thresholds: 250 messages a day per mailbox and 10 a minute. If the API answers 403 with "Sending too fast" or "daily send limit reached", that is OquMail's own cap, not a 421 from the receiver; wait a minute, or until local midnight for the daily limit.

Common questions

Is 421 the same as being blocklisted?

No. A blocklist entry produces a 5xx rejection that names the list. A 421 is a slowdown, and most of them never involve a list at all. Check anyway if the deferrals last more than a day.

How long until it clears?

Usually hours for a one-off burst, two to three days for a reputation problem once the cause is fixed. If nothing changes after a week, the cause is still there.

Can I ask Gmail to lift it?

There is no appeal form for 421 deferrals. Gmail lifts them automatically as the signals improve. Postmaster Tools shows the reputation and spam-rate graphs that drive the decision.

Want to see how your own domain is set up? Use the free SPF checker and DMARC checker: they run in your browser and store nothing.