Short answer: The receiving server isn't accepting your mail right now and asks you to try again later. Gmail mostly returns this for suspicious traffic from an IP, missing reverse DNS (PTR), missing authentication, or TLS problems. Your server retries automatically.
421 4.7.0 bounce message examples
The same code comes with different wording on different servers. In the bounce (non-delivery report) you may see one of these lines:
421 4.7.0 [203.0.113.10] Our system has detected an unusual rate of unsolicited mail originating from your IP address. (Gmail) 421 4.7.0 [203.0.113.10] This message does not have authentication information or fails to pass authentication checks. (Gmail) 421 4.7.0 Try again later, closing connection. 421 4.7.0 TLS required
Type: Temporary error. The sending server keeps the message in its queue and retries automatically.
What causes it?
| Cause | Explanation |
|---|---|
| Sudden volume spike from the IP | Sending many messages in a short time (a bulk campaign, or a compromised account) triggers rate limits. |
| Missing or mismatched reverse DNS (PTR) | If the sending IP has no PTR record, or it doesn't match the HELO name, Gmail defers the mail. |
| Missing authentication | Mail that fails SPF/DKIM is first deferred, then permanently rejected (5.7.26). |
| TLS required | The receiver only accepts encrypted connections and your server doesn't support STARTTLS. |
What to do as the sender
- Check your mail queue: the server retries automatically for hours, and in many cases nothing else is needed.
- If the error persists, have your hosting or IP provider set a PTR record for the server IP that matches the HELO name.
- Check SPF, DKIM and DMARC and fix what's missing.
- Split bulk sends into batches and ramp volume up slowly. If a spike came from a compromised account, change its password immediately.
- Check your IP against blocklists; reputation problems usually show up in several places.
What the receiving side's admin should check
- Enable STARTTLS with a valid certificate on your sending server.
- Add your domain to Google Postmaster Tools to watch your spam rate and compliance status; Gmail expects the spam rate to stay below 0.3%.
Good to know
4xx codes are temporary: the sending server keeps the message in its queue and retries at intervals (on most servers for up to 4-5 days). The user only gets a "could not be delivered" notice when that period runs out. For a deliberate protection technique that uses the same mechanism, see 451 4.7.1 greylisting.
Protect your inbox from spam, malware and phishing: point your MX record to Spambil and try it free for 7 days. No credit card required.
Frequently asked questions
Is mail that got a 421 lost?
No. On a temporary error the message waits in the sending server's queue and is retried automatically. It only bounces if delivery keeps failing for several days.
I only get this error for mail to Gmail. Why?
Gmail's reputation and authentication rules are among the strictest. Check your PTR record, SPF/DKIM/DMARC status and sending volume.