Fix Gmail error 550 5.7.26: sender is unauthenticated

What the error says

Gmail documents this rejection as: This email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with SPF or DKIM. Google's sender guidelines say messages that are not authenticated might be marked as spam or rejected with a 5.7.26 error.

Cause

The message failed (or had no) SPF and DKIM authentication for the domain in the From address. Google requires SPF or DKIM from all senders, and SPF, DKIM and DMARC from bulk senders.

How to fix it

  1. Find your sending services (your mailbox provider, newsletter tool, CRM, invoicing app).
  2. Publish one SPF record listing them. Only one v=spf1 TXT record may exist per domain. 
    example.com.  TXT  "v=spf1 include:_spf.google.com include:sendgrid.net ~all"
  3. Turn on DKIM in each service and publish the key it gives you. 
    selector1._domainkey.example.com.  TXT  "v=DKIM1; k=rsa; p=MIGfMA0GCSq..."
  4. Add a DMARC record, starting in monitoring mode. 
    _dmarc.example.com.  TXT  "v=DMARC1; p=none; rua=mailto:dmarc@example.com"
  5. Wait for DNS to update, then send a test to a Gmail address and use Show original to confirm SPF and DKIM say PASS.

Tip: the authenticated domain should match your visible From domain (alignment), or DMARC will still fail.

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Sources

More email delivery fixes