550 5.7.25: reverse DNS check failed
The sending server's IP address failed the receiver's reverse DNS check, usually because it has no PTR record or the PTR name does not lead back to the IP.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.7.25
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 5.7.25 Reverse DNS validation failed
What it means
Many receivers look up the PTR (reverse DNS) record of the connecting IP address and then check that the name it returns points back to the same IP. RFC 7372 registered X.7.25, with reply 550, for a refusal based on that check, and refers to RFC 7001 for the test itself. It is permanent for this message. A PTR record belongs to whoever controls the IP range, so the fix is made at your hosting provider or ISP, or in your email service's settings, rather than in your own domain's DNS zone.
Common causes
- The sending IP address has no PTR record.
- The PTR name does not resolve back to the same IP (forward-confirmed reverse DNS fails).
- A generic PTR name that the receiver treats as a home or dynamic address.
How to fix it
- 01Look up the PTR record for your sending IP.
- 02Ask your hosting provider or ISP to set a PTR record with a name you control, such as mail.example.com.
- 03Create an A (or AAAA) record for that name pointing back to the same IP.
- 04Use the same name in your server's HELO/EHLO greeting.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.7.1Delivery not authorised.The receiving server refuses the message because this sender is not allowed to send to this recipient, as a result of filtering by host or by recipient.
- 550 5.7.26Multiple authentication checks failed.The message failed more than one authentication check, for example SPF and DKIM, and the receiving server's policy rejects it.
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
5.7.25 at other providers
- 550 5.7.25No PTR record, blocked.Gmail rejected the message because the sending IP has no reverse DNS record, or the hostname in that record does not point back to the IP.
- 5.7.25IPv6 sender without reverse DNS.Microsoft 365 refused mail from an IPv6 address that has no reverse DNS (PTR) record; the same rule also appears as 450 4.7.25.
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce