550 5.7.26: Multiple Authentication Checks Failed
The 550 5.7.26 bounce means more than one authentication check failed. It is a Gmail code, not a Microsoft one, and the sentence beside it names which checks failed.
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550
5.7.26
Multiple Authentication Checks Failed
Very Common
🔢 Enhanced Status Code Breakdown: 5.7.26
| Component | Value | Meaning |
|---|---|---|
| Class | 5 | Permanent failure (Hard bounce) |
| Subject | 7 | Security or policy |
| Detail | 26 | Multiple Authentication Checks Failed |
Per RFC 3463 Enhanced Mail System Status Codes. Class (X) = severity, Subject (Y) = category, Detail (Z) = specific condition.
💬 What This Error Means
Your email failed its 'ID check', and more than one check went down with it. RFC 7372 registers 5.7.26 as "multiple authentication checks failed" and says outright that which mechanisms failed are not specified, so the code alone does not mean DMARC. The sentence beside it does the naming, and at Gmail it is one of three: no authentication at all, an SPF hard fail on -all, or a DMARC policy rejecting unauthenticated mail. Read it before you assume alignment.
Common Causes
- Neither SPF nor DKIM could verify that the email came from your domain
- Your SPF record ends in -all and the sending IP is not authorised by it
- Your email service is "authorized" but it's sending under its own domain instead of yours (alignment failure)
- You're using a third-party service (like a CRM or helpdesk) that hasn't been fully set up with your DKIM keys
How to Fix This
- Read the sentence in the bounce first: it names which checks failed, and the fix differs between them
- Ensure your email service is set up to sign messages with your own domain's DKIM keys
- Check that your SPF record includes all the services you use to send mail
- Use our Domain Checker to see exactly where the "alignment" is failing
- Look at your DMARC reports to see if any legitimate mail is being blocked
📚 Official Documentation
Gmail issues 5.7.26 for three separate situations and names each in its own text: the sender is unauthenticated (neither SPF nor DKIM passes), the MAIL FROM domain has an SPF -all policy that failed, or a DMARC policy rejects unauthenticated mail. Its bulk-sender rules (Feb 2024) require a DMARC record from senders of 5,000+ messages a day, and the policy may be p=none.
📖 Google Workspace / Gmail documentation → Browse all Google Workspace / Gmail error codes →Microsoft does not issue 5.7.26. The code does not appear in Exchange Online's NDR code table, and Microsoft's own outbound-deliverability post quotes the 550-5.7.26 string as one Gmail returns to mail sent from Microsoft 365. Microsoft's own authentication rejection is 550 5.7.515: from May 2025, Outlook.com, Hotmail and Live require SPF and DKIM to pass plus a passing DMARC check from any domain sending 5,000 or more messages a day.
📖 Microsoft 365 / Outlook documentation → Browse all Microsoft 365 / Outlook error codes →📋 Real-World Example Messages
These are real bounce message formats you might receive. Paste yours into the Bounce Decoder for instant analysis.
From: Mail Delivery Subsystem <[email protected]> Subject: Delivery Status Notification (Failure) Status: 5.7.26 Diagnostic-Code: smtp; 550 5.7.26 This mail is unauthenticated, which poses a security risk to the sender and Gmail users and has been blocked. The sender must authenticate with at least one of SPF or DKIM.
🔗 Related Error Codes
🔧 Related Diagnostic Tools
These tools can help you diagnose and fix this type of bounce:
🔧 This Bounce Is Related to Email Authentication
Fixing this type of bounce requires correctly configured SPF, DKIM, and DMARC records. Our free Domain Checker analyses all three in one scan and tells you exactly what to fix.
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