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Why a Valid SPF Record Can Still Silently Fail Your Email Authentication

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A rule embedded in RFC 7208 limits SPF email authentication to a maximum of ten DNS lookups per evaluation, and exceeding that limit causes an outright failure rather than a graceful degradation. A scan of the top 10,000 domains found that 1.7% were already over this threshold, a figure that tends to grow as organizations add more email-sending services. The problem is difficult to detect because the SPF record itself appears syntactically correct and publishes without errors, while the failure only surfaces at the recipient's mail server during send time. Each 'include' directive in an SPF record can trigger multiple recursive lookups, meaning a few common providers like Google and Microsoft can quickly consume most of the ten-lookup budget. Fixes include removing unused includes, flattening stable senders to direct IP entries, using providers' consolidated include records, and splitting senders across subdomains to distribute the lookup count.

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Why a Valid SPF Record Can Still Silently Fail Your Email Authentication · ShortSingh