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ZeroHook | SecurityTools

Error codes

550 5.7.24 The SPF record of the sending domain has one or more suspicious entries.

Gmail blocked the message because it treats one or more terms in the sending domain's SPF record as suspicious.

  • 451 4.7.24 is the rate-limited twin of the same suspicious-SPF case.

The title is the 550 5.7.24 row on Google's SMTP error list, last updated 18 September 2026 UTC. That row does not name the term. The sender guidelines page is the other document on this page.