421 4.7.0: Try Again Later (Gmail Deferrals and Low Reputation)

A 421 4.7.0 from Gmail is a temporary deferral: Gmail won't take your message right now and wants you to try later. Most of the time the reason is in the text that follows the code, and the most common versions mean your sending IP or domain has a very low reputation, your content or links look suspicious, or your IP has no matching reverse DNS. Your server will retry on its own, but the deferrals won't stop until you fix the underlying cause.

The 4 at the start of the code is what makes it temporary. Under RFC 5321, a 4xx reply means "try again"; a 5xx means "don't." That's the difference between this and Gmail's 550 5.7.1 unsolicited mail rejection, and it's often the step before it.

What the bounce says

4.7.0 is a catch-all status, and Gmail uses it for several different problems. Google documents these versions:

421 4.7.0 This message is suspicious due to the very low reputation of the
sending IP address.

421 4.7.0 This message is suspicious due to the very low reputation of the
sending domain.

421 4.7.0 This message is suspicious due to the nature of the content or the
links within.

421 4.7.0 The IP address sending this message does not have a PTR record, or
the corresponding forward DNS entry does not point to the sending IP. To
protect our users from spam, email sent from your IP address has been
temporarily rate limited.

421 4.7.0 TLS required for RCPT domain, closing connection.

421 4.7.0 Try again later, closing connection.

In your logs, the text usually wraps across several lines that each start with 421-4.7.0, and Gmail includes your sending IP in brackets and a link to its help pages. Read the full text, not just the code.

What it means

Gmail scores every message on the reputation of the sending IP, the sending domain, the domains in your links, and how recipients have treated your mail. When those signals are bad enough, Gmail stops accepting at full speed and starts deferring. A deferral gives you a warning and a chance to fix things. Senders who keep pushing the same mail at the same volume often see the deferrals turn into permanent 550 rejections.

The PTR and TLS versions are different: they're configuration problems, not reputation problems, and they clear as soon as the configuration is fixed.

Common causes, most likely first

1. High spam complaint rate. Google tells senders to keep their user-reported spam rate below 0.1% and to avoid ever reaching 0.3%. A complaint spike is the most common route to a low-reputation deferral. See Gmail's spam rate threshold for what those numbers mean in practice.
2. Poor list quality. Old, purchased or scraped lists generate bounces, spam trap hits and complaints all at once.
3. A new or cold IP or domain sending too much, too soon. Gmail has no history to trust, so it throttles.
4. Missing or failing authentication. Unauthenticated mail gets its own codes (550 5.7.26 and the 421 4.7.26 to 4.7.32 range), but weak authentication also drags down domain reputation.
5. Links to low-reputation domains. A shared link shortener or a tracking domain used by other senders can carry their reputation into your mail.
6. No valid PTR record. The sending IP needs reverse DNS that resolves to a hostname, and that hostname needs to resolve back to the same IP. Google's guidelines require this for all senders.
7. TLS not offered when the recipient's domain requires it. This is most common with Google Workspace domains that enforce TLS.

How to fix it, step by step

1. Read the full bounce text. Group your Gmail deferrals by message (IP reputation, domain reputation, content, PTR, TLS) and by sending IP and domain.
2. If it's PTR: set reverse DNS on the sending IP to a hostname you control (for example, mail.example.com) and make sure that hostname's A record points back to 203.0.113.7. Your hosting provider or ESP usually sets the PTR.
3. If it's TLS: enable STARTTLS on your outbound server.
4. If it's reputation or content: check your spam rate and compliance status in Google Postmaster Tools. Google retired the old IP and domain reputation dashboards in 2025, so spam rate and the compliance view are now your main Gmail signals.
5. Confirm authentication. Run your domain through the SPF, DKIM and DMARC checker. Bulk senders to Gmail need SPF, DKIM, DMARC and one-click unsubscribe.
6. Stop the mail that's hurting you. Pause sends to unengaged segments and any recently added list source. Mail your most engaged recipients first.
7. Reduce volume to Gmail. Lower your send rate and ramp back up gradually as deferrals fall.
8. Check your links. Remove public shorteners and test whether a version of the message without a particular link is accepted.
9. Don't fight the retries. Let your MTA retry deferred mail on its normal schedule. Don't rotate to new IPs or domains to get around the deferral. New infrastructure has no reputation, and spreading the same mail across it spreads the problem.

How to confirm it's fixed

The share of Gmail traffic getting 421 responses in your MTA logs should shrink over days. Your spam rate in Postmaster Tools should come down and stay under 0.1%. Check inbox placement with a seed test as well, since Gmail accepting mail isn't the same as Gmail inboxing it.

How to keep it from coming back

• Watch Postmaster Tools spam rate weekly, and per stream if you split mail across subdomains.
• Remove hard bounces immediately and sunset unengaged addresses.
• Warm new IPs and domains slowly.
• Keep reverse DNS, TLS and authentication in your change checklist whenever infrastructure moves.

For the broader picture, see why emails go to spam and our summary of the Gmail and Yahoo bulk sender rules.

When to get help

If Gmail keeps deferring you for low reputation after you've fixed authentication and tightened your lists, or deferrals have started turning into 550 rejections, it's worth getting an experienced set of eyes on your data. Get in touch.

FAQ

Will my email eventually be delivered after a 421 4.7.0?

Often, yes. Your server retries deferred mail automatically and some of it gets through. If the reputation problem continues, messages can run out of retries and bounce.

Is 421 4.7.0 the same as being blocklisted?

No. It's Gmail's own reputation system, not a public blocklist, so no blocklist checker will show it. The bounce text is the only place you'll see it.

How is this different from 421 4.7.28?

421 4.7.28 specifically says Gmail detected an unusual rate of unsolicited mail from your IP, domain or netblock. 421 4.7.0 covers a wider set of problems, so the explanatory text matters more.

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

No. A fresh IP has no reputation, gets throttled anyway, and moving mail to escape filtering makes things worse. Fix what caused the low reputation instead.