@Mail Domain Check

Gmail - SMTP 451 4.7.24 / 550 5.7.24

Fix Gmail 4.7.24 or 5.7.24 suspicious SPF entries

Gmail uses these codes when it considers one or more entries in the sending domain's SPF record suspicious. Check the exact SPF identity named in the rejection before changing DNS.

What the two Gmail codes mean

Google's current SMTP error table gives the same SPF explanation in two forms. 451 4.7.24 is a temporary rate limit, so a normal mail queue may retry later. 550 5.7.24 is a permanent rejection of that message. Repeated retries do not repair either form; the SPF record or the actual sending identity must be investigated.

Gmail does not publish a complete machine-readable list of every entry it may label suspicious. A scanner can prove specific public-DNS faults, but it cannot claim that a clean static record will be accepted by Gmail.

Check the correct SPF identity

  1. Keep the complete SMTP response, including the domain and IP address Gmail names.
  2. Identify the RFC5321 MAIL FROM domain used for SPF. It can differ from the visible From: domain.
  3. Scan that domain, not Gmail's receiving host and not an unrelated website hostname.
  4. Compare every authorized sender with the systems that actually send mail for the domain.

Public SPF faults this check can prove

  • More than one v=spf1 record at the same name, which produces PermError.
  • A missing, duplicated, or cyclic nested include: or redirect= policy; temporary DNS failures are reported as incomplete verification.
  • A potential DNS lookup count above the RFC 7208 limit of 10, or a record already close to that limit.
  • An unrestricted +all, a neutral or rollout policy, a missing terminal policy, or the discouraged ptr mechanism.

These findings are concrete configuration risks. They are not a reverse-engineered list of Google's private abuse or reputation signals.

Repair without widening authorization

  1. Remove services that no longer send for the domain. Do not add the Gmail receiving IP from the bounce.
  2. Correct misspelled or retired include targets and ensure each target publishes exactly one SPF record.
  3. Keep only verified provider includes or owned IP ranges. Treat copied SPF flattening output as stale unless it is automatically maintained and reversible.
  4. Reduce recursive DNS mechanisms before the 10-query ceiling, then choose ~all or -all only after all legitimate senders are covered.
  5. Wait for authoritative DNS to serve the new record, send one new test through the same path, and inspect the new Authentication-Results and SMTP response.

What still requires message evidence

A domain-only scan cannot evaluate SPF for a specific MAIL FROM, HELO identity, sending IP, DNS cache state, or Gmail policy decision. It also cannot prove that every listed provider is still authorized by the domain owner. Use the full rejection and a newly generated message as the final evidence; do not infer success from DNS syntax alone.

Primary sources