SPF Flattening

SPF flattening looks up a domain's current SPF record and recursively resolves every include, a, and mx mechanism down to the underlying ip4/ip6 addresses, producing a flattened record that needs no further DNS lookups to evaluate.

Why it matters

SPF enforces a hard limit of 10 DNS lookups during evaluation; exceeding it causes receiving mail servers to treat the whole record as a permanent error, regardless of whether the mail was actually authorized. Domains that pile up include mechanisms from several email tools often hit this limit without realizing it — flattening removes the lookups at the cost of needing to be regenerated whenever an underlying provider changes its IP ranges.

Examples

A domain using Google Workspace plus two marketing tools can easily accumulate 8-10 nested lookups once each include's own includes are counted.
A flattened record trades lookup-limit safety for staleness risk — if a provider changes its IP ranges without you noticing, the flattened record silently stops matching.
Because of that staleness risk, flattening is usually recommended only once a domain is already close to the 10-lookup limit, not as a default first step.