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Exchange Online 550 5.7.25 — IPv6 Reverse DNS Required

Exchange Online rejected an IPv6 delivery because the sending IPv6 address did not have the reverse-DNS identity required for anonymous inbound mail.

Diagnose your exact bounce Reverse DNS & HELO Auditor

Most common causes

  • The sending IPv6 address has no PTR record
  • The outbound mail path recently moved to IPv6 without matching reverse DNS
  • The PTR exists for a different address than the one used for delivery

What to verify next

  1. Run Reverse DNS & HELO Auditor for the sending IPv6 address
  2. Confirm the outbound IP shown in the rejection or message trace
  3. Publish and verify the appropriate PTR through the IP-space provider
  4. Confirm the forward hostname and SMTP identity are consistent after the change

Best diagnostic path

Live analysis

Mail Failure Doctor

Classify the complete rejection and preserve provider-specific diagnostic context.

Open analysis → Live analysis

Reverse DNS & HELO Auditor

Check reverse identity, forward confirmation and SMTP greeting consistency.

Open analysis → Live analysis

Email Infrastructure Digital Twin

Map the domain’s public mail infrastructure and provider relationships before remediation.

Open analysis →

Why the exact message matters

The same status family can be triggered by different conditions, and providers frequently add diagnostic text that narrows the issue. Use the complete rejection text rather than treating the numeric code as a complete diagnosis.

Provider reference

For the provider-defined meaning and current requirements, review Microsoft Exchange Online NDR reference.