Teaser only. This is not the full article. Complete guide with M365 and Google Workspace DNS steps: Fix Gmail 550 5.7.26 Auth Errors
Payroll mail bounces. The SMTP reply is short and ugly:
550 5.7.26 Authentication Required
Gmail is not guessing about your intentions. It rejected the message because SPF, DKIM, or DMARC alignment failed on the sending path. Outlook might still accept similar mail. Google won't in 2026.
Open the bounce or Show original in Gmail. Look for spf=, dkim=, and dmarc= in Authentication-Results before you touch DNS.
In the full post on zerohook.org:
- How to read the 550 5.7.26 bounce line by line
- SPF/DKIM/DMARC fixes for Microsoft 365 and Google Workspace
- ESP alignment when marketing sends as your domain
- When to check envelope-from vs. From: header
Read the full guide: Fix Gmail 550 5.7.26 Auth Errors
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