SPF or DKIM authentication issue
The receiving server could not verify the sending domain’s authentication.
What to do now
- Check whether SPF authorizes the production sending path.
- Check the DKIM signature and selector public key.
- Check DMARC policy and alignment with the From domain.
Direct SMTP tests from a local PC may produce different SPF/DKIM results from production sending.
Representative reply
550 5.7.26 SPF or DKIM authentication failed
Retry decision
Resend after correcting production authentication.
Why it happens
SPF policy for the sending path or DKIM signature verification may fail.
Typical stage
DATA
- CONNECT
- EHLO
- MAIL FROM
- RCPT TO
- DATA
- DELIVERY
Stages vary by server. Confirm the actual stage from the complete reply and session logs.
What is SMTP Reply Code?Open only the explanations you need
Three-digit server reply: 4xx is transient, 5xx permanent. Read the complete text to investigate the cause.
What is Enhanced Status Code?Open only the explanations you need
An extended status such as 5.1.1. Server wording and usage may vary.
What is SPF?Open only the explanations you need
DNS policy identifying authorized senders for a domain.
What is DKIM?Open only the explanations you need
Verifies a domain signature using a public key.
What is DMARC?Open only the explanations you need
Policy checking alignment of the From domain with SPF or DKIM authentication.
Check directly
- Check whether SPF authorizes the production sending path.
- Check the DKIM signature and selector public key.
- Check DMARC policy and alignment with the From domain.
If you operate or diagnose the mail server, the tools below can help with these checks.
Cautions
Observed DSNs do not map one-to-one to causes. The stage shown is typical; confirm it with the full reply and actual logs.
Direct SMTP tests from a local PC may produce different SPF/DKIM results from production sending.
Similar reply wording and observed status codes
spf or dkimspf softfaildkim signature processing failed
Observed DSNs: 4.7.26, 5.7.26, 5.7.0