451 4.7.23 Gmail bounce: PTR record missing
What 451 4.7.23 means
Gmail temporarily defers mail because the sending IP's PTR record is missing or does not forward-resolve.
Message text
The sending IP address for this message doesn't have a PTR record, or the PTR record's forward DNS entry…
This is an excerpt: the provider's documentation shortens the message. Full text: provider documentation
Common causes of 451 4.7.23
- No PTR record for the sending IP
- PTR record's forward DNS entry does not match the IP
How to fix 451 4.7.23
- Run reverse DNS lookup on the sending IP and fix the PTR.
- Retry after the DNS change has propagated.
Check your domain
Check the reverse DNS (PTR) of your sending IPRuns in your browser; nothing is uploaded.
Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 451 4.7.23
The first part, 451, is the SMTP reply code. Requested action aborted, local error in processing: the server hit a problem and did not finish the command. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
The second part, 4.7.23, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 4 is a Persistent transient failure (4xx): the command was not accepted, but the condition may clear, so the sender should try again later (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 7 is security or policy: authentication, authorization or policy refused the message. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: knowledge.workspace.google.com. Last verified 2026-10-20.
Related guides
- Reverse DNS (PTR) for mail servers: how to set it
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
Related codes
FAQ
What does 451 4.7.23 mean?
Gmail temporarily defers mail because the sending IP's PTR record is missing or does not forward-resolve. This is a temporary failure.
How do I fix 451 4.7.23?
Run reverse DNS lookup on the sending IP and fix the PTR. Retry after the DNS change has propagated.
Is 451 4.7.23 permanent, and should I retry?
451 4.7.23 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.