Gmail 421 4.4.5
Server busy
Gmail 421 4.4.5 means: server busy. It is a temporary failure, so your server retries it. Gmail is busy. Let the queue retry, and if you open many connections to Gmail at once, lower the concurrency.
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
Server busy, 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 | 4.4.5: Mail system congestion |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
What the sender should do
Gmail is busy. Let the queue retry, and if you open many connections to Gmail at once, lower the concurrency.
Would verifying the list have prevented Gmail 421 4.4.5?
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 4.4.5 mean?
- Gmail 421 4.4.5 means: server busy. It is a temporary failure, so your server retries it. Gmail is busy. Let the queue retry, and if you open many connections to Gmail at once, lower the concurrency.
- Should I retry after Gmail 421 4.4.5?
- 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 4.4.5?
- 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.
The same code elsewhere
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