MailFixIt

Gmail temporary reverse-DNS error

Gmail 451 4.7.23: The sending IP address doesn't have a PTR record, or forward DNS doesn't match

Gmail temporarily rate limited the sending IP because PTR is missing or its hostname does not resolve back to that IP. The IP owner must correct reverse DNS.

Problem description

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.
Automated diagnosis: enter the public sending IP. MailFixIt queries its PTR record and confirms that the returned hostname resolves back to the same IP.

Run the automated PTR test

Enter your domain and sending IP. MailFixIt checks PTR and forward DNS automatically and shows the evidence.
Is this temporary or permanent?

451 is temporary. Let the MTA retry with backoff while the IP owner corrects a persistent mismatch.

Manual check

  1. Copy the public sending IP from Gmail's response.
  2. Resolve its PTR hostname.
  3. Resolve that hostname with A or AAAA.
  4. Confirm the original IP is returned.
  5. Check that the final MTA uses a coherent HELO hostname.

How to fix it

Set PTR through the provider that owns the IP, not through an unrelated domain DNS dashboard. Publish matching forward DNS at the hostname you control. For a shared email service, send the complete Gmail response to that provider.

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?

Yes. Enter the sending IP to check PTR and confirm that its hostname resolves back to the same address.

Related email diagnostics

Gmail 550 5.7.25: This message was blocked because the sending IP address doesn't have a PTR record, or forward DNS doesn't reference the sending IP addressFix Gmail 550 5.7.25 by making the sending IP's PTR hostname resolve forward to the same IP. Outlook 550 5.7.25: Access denied, the sending IPv6 address must have a reverse DNS recordFix Microsoft 365 550 5.7.25 when an anonymous sending IPv6 address has no reverse DNS record.

Primary sources: Gmail SMTP errors and codes, Google email sender guidelines.