Exact response pattern
550 5.7.25 Reverse DNS validation failedPermanentGenericDNS
What it means
The sender lacks acceptable reverse DNS or the forward/reverse identity is inconsistent.
Likely causes
- Missing PTR.
- PTR hostname does not resolve back to the IP.
- Generic or mismatched EHLO.
Recommended response
- Set a stable PTR.
- Make the PTR hostname resolve to the sending IP.
- Use a matching EHLO identity.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.