X.4.5 · Network and routing
SMTP enhanced status code 4.4.5
Mail system congestion
X.4.5 means the receiving mail system is congested. It is a temporary error and says nothing about the address or your reputation.
| Registry code | X.4.5 |
|---|---|
| Forms in use | 4.4.5 |
| Registry name | Mail system congestion |
| Subject | 4: Network and routing |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed |
| Associated basic codes | 451 |
| Defined in | [RFC 3463] |
What the registry says
The mail system was unable to deliver the message because the mail system was congested. This is useful only as a persistent transient error.
IANA SMTP Enhanced Status Codes registry, X.4.5
The basic codes it travels with
- 451 Requested action aborted: local error in processing
In practice
X.4.5 says the receiving system is congested. Gmail sends 421 4.4.5 when its servers are busy. Large providers also use congestion replies to slow senders down during traffic spikes. Your server retries on its own. Reducing concurrent connections to the provider helps if it keeps happening. Congestion replies say nothing about the address, so do not remove recipients because of them.
What the sender should do
Let the queue retry, and reduce the number of parallel connections you open to that provider.
Would verifying the list have prevented 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.
How providers use X.4.5
Quoted from each provider’s own error reference.
- Gmail 421 4.4.5
Server busy, try again later. For more information, go to About SMTP error messages.
- Gmail 550 5.4.5
Daily user sending limit exceeded. For more information on Gmail sending limits, go to Gmail sending limits.
Common questions
- What does 4.4.5 mean?
- X.4.5 means the receiving mail system is congested. It is a temporary error and says nothing about the address or your reputation.
- What should I do about 4.4.5?
- Let the queue retry, and reduce the number of parallel connections you open to that provider.
- Would an email verifier have caught 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.
Full guides for this code
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
- Google Workspace Admin Help: Gmail SMTP errors and 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