Gmail 421 4.3.0
Temporary system problem
Gmail 421 4.3.0 means: temporary system problem. It is a temporary failure, so your server retries it. Google's side had a temporary fault. Let your server retry; no change to the message or list is needed.
Gmail’s exact wording
Quoted from Google Workspace Admin Help: Gmail SMTP errors and codes, checked 2026-11-05. Placeholders are Gmail’s own.
Reply text
Temporary System Problem. Try again later. For more information, go to About SMTP error messages.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 421: <domain> Service not available, closing transmission channel (RFC) |
| Enhanced code | 4.3.0: Other or undefined mail system status |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
What the sender should do
Google's side had a temporary fault. Let your server retry; no change to the message or list is needed.
Would verifying the list have prevented Gmail 421 4.3.0?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Gmail 421 4.3.0 mean?
- Gmail 421 4.3.0 means: temporary system problem. It is a temporary failure, so your server retries it. Google's side had a temporary fault. Let your server retry; no change to the message or list is needed.
- Should I retry after Gmail 421 4.3.0?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Gmail 421 4.3.0?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05