Enhanced SMTP status code · RFC 3463 · IANA
5.7.25 Reverse DNS validation failed
The sending server's IP address failed the reverse DNS (PTR) check the recipient requires (RFC 7372).
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
- Associated basic codes (IANA registry)
How 5.7.25 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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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. Gmail requires that sending IP addresses have a PTR record.
The sending IP address has no PTR, or the PTR name does not lead back to the same address, so Gmail limits or blocks its mail. The PTR is set by whoever holds the IP address. Check with SMTP server check.
Microsoft 365 / Exchange Online
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5.7.25
Access denied, the sending IPv6 address [2a01:111:f200:2004::240] must have a reverse DNS record
The sending IPv6 address must have a PTR record. Check with SMTP server check.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026