Gmail bounce 550-5.7.26: what it means and the fix

If your bounces say "550-5.7.26 This mail has been blocked because the sender is unauthenticated", Gmail is rejecting you for one specific, fixable reason: your domain doesn't pass SPF or DKIM (and for bulk senders, DMARC is also required).

Why this started happening

Since February 2024, Google enforces authentication for all senders — and rejects outright rather than filing to spam. Microsoft followed with the same rules for Outlook.com in May 2025. If your domain was set up years ago and "just worked", this is the new rule catching up with you.

The fix, in order

• 1. Check what's actually failing. Don't guess — the error doesn't tell you whether it's SPF, DKIM, or both.
• 2. Publish/repair SPF. One TXT record listing every service that sends as your domain, ending in ~all. Two records, or more than 10 DNS lookups, silently voids it.
• 3. Turn on DKIM at your provider. Google Workspace, Microsoft 365, and every mainstream ESP has a screen that generates the record for you.
• 4. Add DMARC. v=DMARC1; p=none; rua=mailto:you@yourdomain is enough to comply.
• 5. Wait ~15 minutes, send a test to a Gmail address.

The scan shows which of the three is failing on your domain. The $29 Rescue Kit then hands you the exact records to paste — built from the senders we detect on your domain — plus the checklist and a list clean so bounces don't drag you back here.