550 5.7.26: multiple authentication checks failed
The message failed more than one authentication check, for example SPF and DKIM, and the receiving server's policy rejects it.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.7.26
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
550 5.7.26 Multiple authentication checks failed
What it means
RFC 7372 says a receiver that rejects a message after several authentication checks failed should use X.7.26 rather than report only the first failure, and the code does not say which mechanisms failed. RFC 8617 adds that an ARC failure may be reported this way too. RFC 7372 pairs it with reply 550, so the refusal is permanent. In practice it most often means that neither SPF nor DKIM passed in a way that lines up with the From domain, which is also what a DMARC failure looks like. Gmail, for one, uses 550 5.7.26 in its own bounces for unauthenticated mail.
Common causes
- The sending service is not in your SPF record and does not sign with DKIM for your domain.
- SPF and DKIM pass only for the service's own domains, so nothing aligns with your From domain.
- Spoofed mail using your domain, correctly rejected.
How to fix it
- 01Check SPF, DKIM and DMARC for the domain in your From address.
- 02Set up DKIM signing with your own domain in each sending service.
- 03Add every sending service to your SPF record.
- 04Send a test to a mailbox you control and confirm that SPF and DKIM pass and align.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 7372, section 3.4: Multiple Authentication Failures Coderfc-editor.org
- RFC 8617, section 5.2.2: Responding to ARC Validation Failuresrfc-editor.org
- IANA, SMTP Enhanced Status Codes registry (Enumerated Status Codes)iana.org
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
Related codes
- 550 5.7.20No valid DKIM signature.The receiving server requires a valid DKIM signature and found none that passed verification, so it refused the message.
- 550 5.7.23SPF check failed.The SPF check on the sending server's IP address gave a fail result for the sender's domain, and the receiving server's policy rejects such mail.
- 5.7.29ARC validation failed.The message carried an ARC chain, the record that forwarders add when they pass mail on, and the receiver found that chain invalid.
- 5.7.1Delivery not authorised.The receiving server refuses the message because this sender is not allowed to send to this recipient, as a result of filtering by host or by recipient.
5.7.26 at other providers
- 550 5.7.26Unauthenticated sender, blocked.Gmail rejected the message because it passed neither SPF nor DKIM, and Gmail requires every sender to pass at least one.
- 550 5.7.26SPF hard fail.Gmail rejected the message because the sending IP is not allowed by the SPF record of the envelope domain, and that record ends in -all.
- 550 5.7.26Rejected under the sender's DMARC policy.Gmail rejected the message because it failed DMARC, and the From domain's DMARC policy tells receivers to refuse mail that fails.
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce