Enhanced Status Code 4.3.0: Other Mail System Status - Temporary
Enhanced Status Code 4.3.0 means “Other Mail System Status - Temporary.” A temporary problem inside the receiving mail system that it has not described further. Gmail uses it for general temporary rejections and for a transaction addressing multiple destination domains.
451 4.3.0 Multiple destination domains per transaction is unsupported. Please try againWhat does 4.3.0 mean?
Enhanced status code 4.3.0 is the catch-all for the mail system class in its temporary form. RFC 3463 describes X.3.0 as an undefined mail system status, meaning the receiving system had a problem of its own rather than one caused by the message, the recipient or the network.
Google returns it in two distinct shapes. As 451 4.3.0 it means the mail server temporarily rejected the message, which is a general deferral with no further detail. Also as 451 4.3.0 it appears with text saying multiple destination domains per transaction are unsupported, which is a genuine and fixable protocol issue: some senders try to deliver to recipients at several different domains within a single SMTP transaction, and Google refuses that. As 421 4.3.0 it means a temporary system problem and asks you to retry.
The multiple destination domains case is the only one of these with a concrete fix, and it is a real bug in some homegrown senders. Group recipients by destination domain and open a separate transaction for each. The others are genuinely the receiver's problem and resolve on retry.
How 4.3.0 plays out
4.3.0 replyWhere 4.3.0 sits: soft vs hard bounce
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| SMTP class | 4xx | 5xx |
| What to do | Let it retry | Suppress the address |
| Recoverable? | Often | No |
| 4.3.0 is | ✓ this code |
What each provider means by 4.3.0
A registered code does not oblige a provider to use it that way, and the large ones diverge. Match the wording in your own bounce, not the definition above.
Common causes of 4.3.0
- A transient internal fault at the receiving mail system
- The transaction addressed recipients at more than one destination domain
- The receiving system is restarting or partially degraded
- A backend dependency at the receiver, such as storage or a directory, is unavailable
- A temporary resource shortage the server did not classify further
- Maintenance in progress at the destination
How to fix 4.3.0
- Read the text, since only the multiple destination domains case is fixable by you
- Group recipients by destination domain and use one transaction per domain
- Let the queue retry for the general cases, which clear without intervention
- Confirm your retry schedule uses exponential back-off
- Check the provider status page if the condition persists for hours
- Distinguish this from 4.4.x, which is network rather than mail system