5.7.26 SMTP bounce: authentication failed
What 5.7.26 means
The message failed more than one authentication check. The specific failing mechanisms are not named in the code.
Message text
Multiple authentication checks failed
Common causes of 5.7.26
- Both SPF and DKIM failed for the message
- Message failed DMARC alignment
How to fix 5.7.26
- Run SPF checker, DKIM checker and DMARC checker on the From domain.
- Make at least one of SPF or DKIM pass and align with the From domain.
- Use email header analyzer to see which checks failed.
Check your domain
Check the SPF record of your sending domainRuns 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 5.7.26
The second part, 5.7.26, 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: www.rfc-editor.org. Last verified 2026-10-20.
Related guides
- DMARC fails even though SPF and DKIM pass: fixing alignment
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
Related codes
- 550 5.7.26 Gmail: blocked by dmarc policy
- 550 5.7.26 Gmail: no spf or dkim pass
- 550 5.7.26 Gmail: SPF hard fail
- 5.7.0 SMTP: security status
- 5.7.1 SMTP: delivery not authorized
- 5.7.23 SMTP: SPF validation failed
FAQ
What does 5.7.26 mean?
The message failed more than one authentication check. The specific failing mechanisms are not named in the code. This is a permanent failure.
How do I fix 5.7.26?
Run SPF checker, DKIM checker and DMARC checker on the From domain. Make at least one of SPF or DKIM pass and align with the From domain. Use email header analyzer to see which checks failed.
Is 5.7.26 permanent, and should I retry?
5.7.26 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.