Gmail temporary reverse-DNS error
Fix Gmail 451 4.7.23 PTR rate limiting
Gmail uses 451 4.7.23 when the sending IP lacks PTR or its PTR hostname does not resolve back to the IP. Delivery is temporarily rate limited, but the IP owner should correct reverse DNS.
Gmail reports:
451 4.7.23 The sending IP address doesn't have a PTR record, or the PTR record's forward DNS entry doesn't match the sending IP address.
451 is temporary. Let the MTA retry with backoff, while correcting a persistent identity mismatch before increasing volume.
Manual check
- Copy the public sending IP from Gmail's response.
- Resolve its PTR hostname.
- Resolve that hostname with A or AAAA.
- Confirm the original IP is returned.
- Check that the final MTA uses a coherent HELO hostname.
How to fix it
Set PTR through the IP-owning provider, not through an unrelated domain DNS dashboard. Publish matching forward DNS at the hostname you control. Shared email services should manage this; escalate failures to that provider with the complete Gmail response.
Frequently asked questions
Is Gmail 451 4.7.23 temporary?
Yes. Preserve normal retry with backoff, but fix persistent PTR or forward-DNS mismatch.
Who controls PTR?
The provider or network operator that owns the sending IP address.
Does MailFixIt currently test reverse DNS?
No. Follow the manual IP, PTR, and forward-confirmation checks on this page.
Related email diagnostics
Primary sources: Gmail SMTP errors and codes and Google email sender guidelines.