550 5.7.25: reverse DNS validation failed
The sending server's IP address failed a reverse DNS check: it has no PTR record, or the PTR name doesn't resolve back to the same address.
550 5.7.25 This message was blocked because the sending IP address doesn't have a PTR record, or the forwarding DNS entry doesn't reference the sending IP address.
Start of Gmail's text, as listed in Google's SMTP errors reference.
What 550 5.7.25 means
RFC 7372 registered X.7.25, "Reverse DNS validation failed": the SMTP client's IP address failed a reverse DNS validation check, contrary to local policy.
Gmail's sender guidelines say the public IP address of a sending server must have a PTR record that resolves to a host name, and that the same host name must have an A (IPv4) or AAAA (IPv6) record pointing back to the same IP. That two-way match is called forward-confirmed reverse DNS. Gmail rejects with 550 5.7.25 when it's missing, and documents temporary versions too (421 4.7.0 and 451 4.7.23). Microsoft 365 documents 5.7.25 for mail sent over IPv6: "the sending IPv6 address must have a reverse DNS record".
A PTR record isn't set in your domain's DNS zone. It belongs to whoever controls the IP address: your hosting provider, cloud provider or ISP. You set it in their control panel or ask them to.
Likely causes
- The sending IP address has no PTR record.
- The PTR name doesn't resolve back to the same IP (no forward confirmation).
- A new server or IPv6 address was added without reverse DNS.
How to fix it
If you sent the message
- Look up the PTR record for your sending IP and check it resolves back to the same address.
- Set the PTR in your hosting or cloud provider's panel, or ask your ISP, using a name you control.
- Create the matching A or AAAA record for that name.
- If you can't set reverse DNS, send through a provider or relay that has it.
If you're the recipient or their admin
- No action needed, unless you run the receiving server and want to review its reverse DNS policy.
Frequently asked questions
Where do I set a PTR record?
With whoever owns the IP address. For a cloud server or VPS it's usually a reverse DNS setting in the provider's panel; for a business internet line, ask your ISP. You can't set it in your own domain's DNS zone.
Does the PTR name have to match my domain?
Gmail requires that the PTR name resolves back to the same IP. Using a host name on a domain you control, such as mail.yourcompany.com, is clearer than a provider's generic name.
Sources
- RFC 7372: Email Authentication Status Codes
- Email sender guidelines (Gmail Help)
- Gmail SMTP errors and codes (Google Workspace Admin Help)
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn)
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
Related codes
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