4.7.26 Microsoft bounce: ipv6 needs spf or dkim
What 4.7.26 means
Exchange Online will not accept mail sent over IPv6 unless it passes SPF or DKIM, and this message did neither.
Message text
Access denied, a message sent over IPv6 [address] must pass either SPF or DKIM validation, this message is not signed
Common causes of 4.7.26
- Message sent from an IPv6 address that SPF does not authorise
- Message not signed with DKIM
How to fix 4.7.26
- Run SPF checker and make sure the sending IPv6 address is listed in the record.
- Sign outgoing mail with DKIM, then confirm the signature with DKIM checker.
Check your domain
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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 4.7.26
The second part, 4.7.26, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 4 is a Persistent transient failure (4xx): the command was not accepted, but the condition may clear, so the sender should try again later (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.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 421 4.7.26 Gmail: unauthenticated rate limit
- 451 4.7.26 Gmail: dmarc lookup failed
- 4.7.0 SMTP: security status
- 421 4.7.0 Gmail: no PTR record
- 451 4.7.23 Gmail: PTR record missing
- 421 4.7.28 Gmail: rate limited
FAQ
What does 4.7.26 mean?
Exchange Online will not accept mail sent over IPv6 unless it passes SPF or DKIM, and this message did neither. This is a temporary failure.
How do I fix 4.7.26?
Run SPF checker and make sure the sending IPv6 address is listed in the record. Sign outgoing mail with DKIM, then confirm the signature with DKIM checker.
Is 4.7.26 permanent, and should I retry?
4.7.26 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.