# 550 5.7.25: reverse DNS validation failed

URL: https://iblockspam.com/smtp-errors/550-5-7-25
Last updated: 2026-10-09

Code: 550 5.7.25 (permanent failure, the message bounced)
Provider: Standard (RFC)

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.

What the bounce says:

```
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.

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.

## Common 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 (sender)

- 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.

## How to fix it (recipient or admin)

- No action needed, unless you run the receiving server and want to review its reverse DNS policy.

**Q: 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.

**Q: 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: https://www.rfc-editor.org/rfc/rfc7372
- Email sender guidelines (Gmail Help): https://support.google.com/mail/answer/81126
- Gmail SMTP errors and codes (Google Workspace Admin Help): https://knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn): https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/non-delivery-reports-in-exchange-online

---

Source: iBlockSpam, https://iblockspam.com/smtp-errors/550-5-7-25 (this is a Markdown copy; the HTML page is canonical). Quote with attribution and a link. Site index for AI assistants: https://iblockspam.com/llms.txt