450 4.7.25: IPv6 Sender Needs a Reverse DNS (PTR) Record
The 450 4.7.25 deferral means Microsoft 365 got your mail over IPv6 from an address with no valid reverse DNS (PTR) record. Here is what Microsoft documents and what to fix.
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450
4.7.25
IPv6 Sender Needs a Reverse DNS (PTR) Record
Common
🔢 Enhanced Status Code Breakdown: 4.7.25
| Component | Value | Meaning |
|---|---|---|
| Class | 4 | Temporary failure (Soft bounce) |
| Subject | 7 | Security or policy |
| Detail | 25 | IPv6 Sender Needs a Reverse DNS (PTR) Record |
Per RFC 3463 Enhanced Mail System Status Codes. Class (X) = severity, Subject (Y) = category, Detail (Z) = specific condition.
💬 What This Error Means
Microsoft 365 deferred your message because it arrived over IPv6 from an address with no valid reverse DNS (PTR) record. Microsoft documents this for IPv6 only. Anonymous inbound email over IPv6 is opt-in for a Microsoft 365 organisation, and once the organisation has opted in, the source IPv6 address must have a valid PTR record that lets Microsoft find the domain name from the address. Your server retries on its own, but every retry meets the same condition until the PTR record exists.
Common Causes
- The message was sent over IPv6 and the sending IPv6 address has no valid reverse DNS (PTR) record
- The receiving Microsoft 365 organisation has opted in to anonymous inbound email over IPv6, so the PTR requirement applies to mail sent to it
How to Fix This
- Ask whoever controls the sending IPv6 address (your hosting provider or ISP) to publish a PTR record for it
- Check the record with our Reverse DNS tool once it is published
- Microsoft lists a second IPv6 deferral on the same page, 450 4.7.26, for a sender whose domain passes neither SPF nor DKIM. Check both once the PTR record is in place
📚 Official Documentation
Microsoft documents 450 4.7.25 as the error you get after your organisation opts in to anonymous inbound email over IPv6, when the source IPv6 address has no valid reverse DNS (PTR) record: "Service unavailable, sending IPv6 address [...] must have reverse DNS record". Its NDR table lists the permanent form as 5.7.25.
📖 Microsoft 365 / Outlook documentation → Browse all Microsoft 365 / Outlook error codes →📋 Real-World Example Messages
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450 4.7.25 Service unavailable, sending IPv6 address [2001:db8::25] must have reverse DNS record.
🔗 Related Error Codes
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