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Gmail

550 5.7.26 “the sender is unauthenticated”

Gmail checked your message's SPF and DKIM and neither passed. Every sender to Gmail now needs at least one of them. A 550 is a rejection; a 421 slows your mail down for the same reason.

Codes: 550 5.7.26 or 421 4.7.26. Source: Google Workspace: Gmail SMTP errors and codes.

What usually causes it

  • No SPF record, or the service that sent this message isn't listed in it.
  • The message wasn't DKIM-signed, or the signing key isn't published.
  • An SPF record that is broken: two records, or more than 10 DNS lookups.
  • A website form, CRM or printer sending as your domain through a server nobody added to SPF.

How to fix it

  • Publish one SPF record that lists every service sending as you, for example v=spf1 include:_spf.google.com ~all.
  • Turn on DKIM signing in each service that sends as you, and publish its key at <selector>._domainkey (2048-bit where offered).
  • Compare the IP address in the bounce with your SPF record: the missing sender is usually there.
The free email check can find this

The free check shows a missing or broken SPF record and whether DKIM keys are published under the common names. It can't see whether this particular message was signed.

Run the free email check

Want it fixed? Our email deliverability packages start at $49 for an audit and plan. Other error codes.