5.7.25 Microsoft bounce: IPv6 needs reverse DNS

What 5.7.25 means

Exchange Online requires a reverse DNS (PTR) record for an IPv6 address before it accepts mail from it.

Message text

Access denied, the sending IPv6 address [...] must have a reverse DNS record

Common causes of 5.7.25

• Sending IPv6 address has no PTR record

How to fix 5.7.25

1. Run reverse DNS lookup on the IPv6 address and add a PTR record.
2. If you cannot set a PTR, send over IPv4 instead.

Check your domain

Check the reverse DNS (PTR) of your sending IPRuns in your browser; nothing is uploaded.

Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.

How to read 5.7.25

The second part, 5.7.25, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 7 is security or policy: authentication, authorization or policy refused the message. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.

Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.

Official source: learn.microsoft.com. Last verified 2026-10-20.

Related guides

• Reverse DNS (PTR) for mail servers: how to set it
• How to create an SPF record (syntax and examples)
• How DKIM works and how to publish a DKIM record

Related codes

• 5.7.25 SMTP: reverse DNS failed
• 550 5.7.25 Gmail: no PTR record
• 5.7.0 SMTP: security status
• 5.7.1 SMTP: delivery not authorized
• 5.7.23 SMTP: SPF validation failed
• 5.7.26 SMTP: authentication failed

FAQ

All bounce codes