Skip to content
ZeroHook | SecurityTools

Error codes

550 5.7.26 This email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM.

Gmail blocked the message because the sender was not authenticated. It requires SPF or DKIM.

  • The (E)MAIL FROM domain has an SPF record with a hard fail policy (-all) but it fails to pass SPF checks.
  • Unauthenticated email is not accepted due to the domain's DMARC policy.
  • 421 4.7.26 is the rate-limited twin of the same unauthenticated case.

Google prints three different 550 5.7.26 blocks. The title is the first. The two lines under the answer are the other two, from the same help page, last updated 18 September 2026 UTC. Sender guidelines add that all senders need SPF or DKIM, bulk senders need SPF, DKIM, and DMARC, and on direct mail the From domain has to align with the SPF domain or the DKIM domain.