Short answer: Gmail refused the message because it was not authenticated with SPF or DKIM, or because the sending domain's DMARC policy requires rejection. Since 2024 it has been the most visible consequence of Google's sender requirements.
550 5.7.26 bounce message examples
The same code comes with different wording on different servers. In the bounce (non-delivery report) you may see one of these lines:
550 5.7.26 This mail has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. 550 5.7.26 Unauthenticated email from example.com is not accepted due to domain's DMARC policy.
Type: Permanent error. The sending server won't retry; fix the problem and send the message again.
What causes it?
| Cause | Explanation |
|---|---|
| Neither SPF nor DKIM passes | The sending server is not in the SPF record and the message has no valid DKIM signature. |
| DMARC alignment fails | Even if SPF or DKIM technically passes, DMARC fails when the authenticated domain doesn't match the From domain, for example when a newsletter tool signs with its own domain. |
The domain's DMARC policy is p=reject | You, or the owner of the domain being impersonated, asked receivers to reject unauthenticated mail. |
| Someone is spoofing your domain | If you see this error in bounces for mail you never sent, DMARC is doing its job. |
What to do as the sender
- Check your SPF, DKIM and DMARC records.
- Enable DKIM at your mail provider: Google Workspace, Microsoft 365, Zoho Mail, Yandex 360, cPanel.
- If you use a newsletter or marketing platform, turn on DKIM signing with your own domain (often called domain authentication) there; that's the only way to get alignment.
- If you have no DMARC record, start with
v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.comand use the reports to find missing sources.
What the receiving side's admin should check
- Before moving DMARC to
p=reject, confirm in your reports that every legitimate source passes. - If users forward mail, use a setup that doesn't break DKIM signatures on forwarded messages.
Good to know
Since February 2024, Google and Yahoo require SPF or DKIM from all senders, and SPF, DKIM and DMARC from senders of more than 5,000 messages a day. Microsoft has applied similar rules to bulk mail sent to Outlook.com, Hotmail and Live addresses since May 2025; its rejection code for non-compliant bulk mail is 550 5.7.515. More in why are my emails going to spam?
Protect your inbox from spam, malware and phishing: point your MX record to Spambil and try it free for 7 days. No credit card required.
Frequently asked questions
I send very little email. Do these rules apply to me?
Yes. The DMARC requirement is for bulk senders (5,000+ a day), but Gmail requires at least SPF or DKIM from every sender. Without either, you'll get this error.
I get this error for messages I never sent. What's going on?
Someone is sending spam with your domain in the From address and Gmail rejects it because of your DMARC policy; the bounces come back to you. It shows DMARC is working; nothing else is needed.