5.3.5 bounce: system incorrectly configured
5.3.5System incorrectly configuredPermanent: retrying will not help
451 4.3.5 Server configuration error
What it means
RFC 3463 names X.3.5 "System incorrectly configured". A mail server knows that the failure is caused by its own setup, not by your address or your message. The example is the exact text Postfix sends when a lookup table, a policy service, or a restriction is broken; it uses the temporary 4.3.5 so senders retry while the admin fixes it.
The permanent 5.3.5 tends to come from systems that know retries will not help, for example a server that tries to deliver mail to itself because the domain is not in its local domain list, or a relay with no route configured for a domain. Some sending services also report 5.3.5 when the account is missing a required setting. Compare 5.4.6, which is specifically a routing loop, and 5.3.0, which is a system fault without a stated cause.
If the code comes from your own outgoing server, the configuration problem is yours. If the Reporting-MTA or the remote host is theirs, only the recipient’s admin can resolve it.
Likely causes
- A missing or unreadable lookup table, such as a Postfix map that was edited but never rebuilt with `postmap`.
- A policy daemon or milter the server depends on is not running, so the server cannot evaluate its rules.
- The receiving server lacks the recipient domain in its accepted or local domain list.
- A relay has no route or transport configured for the destination domain.
How to fix it
- Read the Reporting-MTA and Remote-MTA fields in the DSN to see which server reports the misconfiguration.
- If it is the recipient’s server, send them the bounce with time and message ID and wait for a fix; your queue retries a 4.3.5 on its own.
- If it is your server, check the MTA log right before the error. Postfix logs the exact table or service it could not use.
- Rebuild changed maps, restart dependent policy services, and run a config check (for example `postfix check`) before reloading.
Checks that apply
- SMTP Diagnostics Connects to the server named in the bounce and shows whether it greets normally or fails before any mail is sent.
- MX Lookup Confirms the domain’s MX hosts, helping you decide whether the misconfigured server belongs to the recipient or to a relay.
FAQ
- Is a 4.3.5 going to deliver eventually?
- Yes, if the admin fixes the configuration before your queue gives up, usually several days.
- Can a sender trigger 5.3.5?
- Not directly. It reports a server setup problem, although a specific domain or address may be what exposes it.