What it looks like in your logs
550-5.7.26 Unauthenticated email from example.com is not accepted due to domain's DMARC policy. Please contact the administrator of example.com domain if this was a legitimate mail. - gsmtpWhat does 5.7.26 mean?
RFC 7372 defines X.7.26 as multiple authentication checks failed. In practice it is the DMARC code. DMARC passes only when SPF or DKIM passes and the domain it passed for aligns with the domain in the visible From header. If neither aligned result exists and the From domain publishes p=reject, the receiver refuses the message and reports 5.7.26. Your server will not retry it.
The receiver can also enforce its own rules. Since February 2024 Gmail requires every sender to pass SPF or DKIM and bulk senders (around 5,000 or more messages a day to Gmail) to pass DMARC as well. Mail that falls short can be rejected with 5.7.26 even if your DMARC policy is only p=none. Gmail treats this as a baseline requirement, not a reputation judgment, so a clean list does not protect you if authentication is missing.
Gmail uses two main texts: "Unauthenticated email from [domain] is not accepted due to domain's DMARC policy" and a message saying the mail lacks authentication information or fails authentication checks. Microsoft Outlook, under its 2025 high-volume sender rules, rejects failing bulk mail with its own 5.7.515 code. Yahoo applies similar requirements with its own codes.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 5.7.26 | ✓ this code |
Common causes
- Neither SPF nor DKIM passed for the message.
- SPF passed, but for your ESP's return-path domain, which does not align with your From domain.
- DKIM passed, but the d= signing domain is the ESP's domain rather than yours.
- Your domain publishes DMARC p=reject and a sender you use is not configured for it.
- The message was forwarded or altered by a mailing list, breaking SPF and the DKIM signature.
- You send in bulk to Gmail without a DMARC record at all.
How to fix 5.7.26
- Check your DMARC record with the DMARC checker; if you have none, create one with the DMARC generator.
- Set up DKIM signing with your own domain for every sending service and verify it with the DKIM checker.
- Configure a custom return-path (bounce domain) on your subdomain so SPF aligns, and check it with the SPF checker.
- Send a test message and read the Authentication-Results header in the header analyzer to see which check failed.
- Review DMARC aggregate reports to find any sender that still fails, before tightening policy further.
- Follow the email authentication guide for the full SPF, DKIM and DMARC rollout order.
5.7.26 questions
What does 550 5.7.26 unauthenticated email is not accepted mean?
It means Gmail rejected the message because it did not pass DMARC for the From domain, and the domain has a reject policy. Either SPF and DKIM both failed, or they passed for a domain that does not match your From address. Fix alignment and resend.
How do I fix the 5.7.26 error in Gmail?
Make sure at least one of SPF or DKIM passes for your own domain. The fastest route is usually DKIM signing with your domain at your ESP. Then confirm your DMARC record with the DMARC checker and test a message with the header analyzer to see an aligned pass.
Can forwarding cause a 5.7.26 failure?
Yes. Forwarding changes the sending IP, so SPF fails, and mailing lists that edit subjects or footers break the DKIM signature. If both fail, DMARC fails. Senders can reduce this with DKIM on every message; receivers can use ARC to trust the forwarder.
What is the difference between 5.7.26 and 5.7.1?
5.7.26 is specifically an authentication failure: fix your records and it goes away. 5.7.1 is a broader policy refusal that can come from reputation, content, blocklists or recipient settings, even when authentication passes. Fix authentication first, then look at other causes.