Gmail and others use 5.7.26 when a message fails authentication: it did not pass SPF or DKIM, or it failed DMARC for the From domain. Since 2024 Gmail and Yahoo reject much more unauthenticated mail.
Policy rejection · fixable
Common causes
- No SPF record, or the sending service is missing from it
- DKIM signing not enabled at your email provider
- DMARC is set to quarantine/reject and neither SPF nor DKIM aligns with the From domain
- Mail sent through a new tool (CRM, newsletter, website form) that was never added to your DNS
How to fix it
- Run the Domain Checker to see which of SPF, DKIM and DMARC fails
- Add every service that sends as your domain to SPF (one record only)
- Turn on DKIM in each sending service and publish its DNS records
- Make sure the From domain matches the domain that passes SPF or DKIM (alignment)
Domain CheckerRun the right check for your domain or paste the full bounce message for a precise explanation.
Open Domain Checker →Frequently asked questions
How do I fix 550 5.7.26 in Gmail?
Authenticate your domain: valid SPF including your sending service, DKIM signing enabled, and a DMARC record. Then resend.
Does DMARC p=none cause 5.7.26?
No — p=none only monitors. The rejection comes from failing authentication; a stricter DMARC policy makes failures more likely to be rejected.