A 421 error is a temporary deferral: the receiving server is telling your server "not now, try again later". With Gmail, 421 4.7.28 is the one to take seriously: it means Gmail noticed an unusual rate of unsolicited mail coming from you and slowed you down on purpose.
Nothing is lost yet, since your server retries. But it is a warning that comes before blocks and spam-folder placement. For every other code, see our guide to SMTP bounce codes.
The common 421 messages
| Code | What it usually means |
|---|---|
421 4.7.28 |
Gmail detected an unusual rate of unsolicited mail from you and temporarily rate limited your mail. The message names what it is counting: your sending IP, or your DKIM, SPF or link domain. |
421 4.7.26 |
Gmail rate limits your mail because it is not authenticated. Fix SPF and DKIM: see 550 5.7.26. |
421 4.7.0 |
A general "try again later" for security or policy reasons. Read the text: it can mean a TLS problem, a sending IP that is not allowed, or suspicious activity. |
421 4.3.2 / 421 4.3.0 |
The receiving server is busy or under maintenance. Nothing to do. |
What it means
- 421: "service not available", a temporary refusal. The connection is closed and your server will try again.
- 4.7.x: a security or policy reason (
.7), but temporary (4).
So the receiving server is throttling you, not refusing you for good.
Why Gmail sends 421 4.7.28
Gmail tracks how people react to your mail per sender identity (IP, DKIM domain, SPF domain, link domains). The usual triggers:
- A sudden spike in volume, for example a new domain or IP sending thousands of messages on its first days.
- Many complaints: people marking your messages as spam.
- Many messages to addresses that do not exist, which looks like a bought or scraped list.
- Low engagement: lots of mail that nobody opens.
How to fix it
- Do not force it. Your server retries on its own. Resending manually or switching IPs to get around the limit usually makes it worse.
- Slow down. Cut your sending volume to Gmail, spread sends over the day, and increase gradually (warm-up) over days or weeks.
- Clean your list before the next send. Remove addresses that bounced, people who never open, and anything old or bought. Verify the list so no invalid addresses go out.
- Check authentication. SPF, DKIM and DMARC should pass and align. Use the SPF and DMARC checker and the DKIM checker.
- Make unsubscribing easy, with one-click unsubscribe for marketing mail, so people leave your list instead of reporting you.
- Watch your spam rate in Google Postmaster Tools and keep it well under the limits in Gmail's sender requirements.
Frequently asked questions
Is 421 4.7.28 a bounce?
Not yet. It is a deferral: your server keeps trying for a while (often up to a few days). If the limit lasts longer than that, the message eventually bounces.
How long does Gmail rate limiting last?
There is no fixed time. It eases as Gmail sees better behaviour from your domain or IP: lower volume, fewer complaints, cleaner lists.
Will a new IP or domain fix it?
Not for long. Moving to a new IP or domain without fixing the cause repeats the problem, and new IPs and domains have no reputation to protect you.