421 4.7.0 is a temporary SMTP reply meaning temporary deferral / rate limited. 4xx codes are temporary. 421 means "service not available, closing channel" and 4.7.0 indicates a policy-based throttle. Your mail server should automatically retry on a backoff schedule.
What 421 4.7.0 means
4xx codes are temporary. 421 means "service not available, closing channel" and 4.7.0 indicates a policy-based throttle. Your mail server should automatically retry on a backoff schedule.
Common causes
You're sending too fast for your reputation
Most commonNew IPs or domains, or a sudden volume spike, get throttled until the receiver builds trust. This is the most common cause.
The receiving server is temporarily overloaded
CommonGenuine temporary resource limits on the receiver's side; these clear on retry.
Greylisting
CommonSome servers defer the first attempt from an unknown sender on purpose and accept the retry.
How to fix 421 4.7.0
- 1
Let your server retry — don't panic
A properly configured mail server retries 4xx deferrals automatically. A single 421 4.7.0 that later delivers is not a problem.
- 2
Warm up new IPs and domains gradually
Ramp volume slowly over days/weeks so receivers build trust. Sending full volume from a cold IP guarantees throttling.
- 3
Fix authentication and reputation if it persists
Sustained 421s across recipients point to a reputation problem — verify SPF/DKIM/DMARC and check blacklists.
Confirm your domain is set up correctly
Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.