Gmail unverified sender warning
Gmail shows unverified sender when it cannot verify your domain on the message. Usually missing DMARC, unsigned DKIM, or SPF/DKIM that does not align with the From: address. Marketing and transactional mail from ESPs triggers this most often when only SPF exists on the root domain.
Three steps to fix it
Publish DMARC at _dmarc.yourdomain.com starting with v=DMARC1; p=none; rua=mailto:... and relaxed alignment while you fix senders.
Enable DKIM on M365, Google Workspace, or your ESP and publish selector CNAME/TXT records in Cloudflare or your DNS host.
Complete ESP domain authentication so test messages to Gmail show dkim=pass and dmarc=pass in Authentication-Results.
Verify the fix once it's live.
Check DMARC setup confirms the change is correct before you move on.
FAQ
Common questions
Does p=none stop the unverified sender banner?+
Not by itself. You still need passing SPF or DKIM with alignment. p=none lets you monitor failures while fixing senders before moving to quarantine or reject.
Is this the same as 550 5.7.26?+
Related but different. Unverified sender is a mailbox warning; 550 5.7.26 is a hard SMTP rejection. Both stem from authentication gaps.
How do I fix this for Mailchimp or SendGrid?+
Enable domain authentication in the ESP dashboard, add their CNAME/TXT records to DNS, and confirm DKIM d= matches your From domain.