451 4.3.0 Temporary system problem / temporary lookup failure
Something inside the receiving server failed while it was handling your message and it is asking you to retry. The 4.3.x class means "mail system" problems: a database, a lookup table, a disk, a dependent service. It is almost never about your message.
What the server replies
451 4.3.0 <chris.meyer@example.com>: Temporary lookup failure
- Sent by
- Any mail server
- Type
- TemporaryRetry helps
- About
- Connection
- Enhanced code
- 4.3.0
Who sends it
Postfix returns "Temporary lookup failure" when an LDAP, MySQL or other table lookup fails mid-session; Exim says "Temporary local problem"; Gmail uses "451 4.3.0 Mail server temporarily rejected message" during internal hiccups; Exchange returns 4.3.0 with "Temporary server error" when a mailbox database is briefly offline.
Why it happens
- A directory or database the mail server consults is down or timing out.
- The server's disk is full or its queue directory is unwritable.
- A content filter or milter the server calls did not respond.
- Routine maintenance on a provider's back end.
How to fix it
Do nothing for an hour
This is the server's problem. Normal retries deliver the message when the fault is fixed, which is usually minutes.
Check whether it is everyone or just you
If every message to that domain is deferred, their system is unwell. If only some are, look at the recipients: a lookup that fails only for certain addresses points at a bad directory entry.
Admin: read the receiving server's log
Postfix logs the real error (for example "connect to ldap server failed" or "No space left on device") next to the 451 it sent. Fix that and the deferrals stop.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 421 Service not available, closing transmission channelThe server is ending the session and wants you to come back later.
- 450 4.1.1 Recipient address rejected: unverified address / mailbox unavailableThe server could not confirm the recipient exists right now and is asking you to try again.
- 451 4.7.1 Greylisted, please try again laterThe receiving server has never seen this combination of sending IP, sender address and recipient before, so it refuses the first attempt and remembers the triple.
Sources
Fewer bounces to look up.
Faivelo queues temporary failures and retries them on a schedule receivers accept, then tells your application once, in plain words, if a message finally cannot be delivered.
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Other free tools
All tools- Email DNS checkerCheck MX, SPF, DKIM, DMARC and reverse DNS for any domain and get the exact records to add or fix.
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- Email header analyzerPaste raw headers to see every hop, where the delay was, and whether SPF, DKIM and DMARC passed.
- Email address validatorCheck whether an address is well formed, whether its domain accepts mail, and whether it is disposable or a role account.