Glossary · Authentication
SPF flattening replaces include mechanisms with the IP addresses they resolve to, cutting DNS lookups to get a record back under the 10-lookup limit.
An ip4: or ip6: mechanism costs no lookups, so swapping include: mechanisms for the ranges they currently resolve to can bring an over-limit record back into compliance.
The catch is drift: providers change their sending IPs, and a flattened record doesn't follow. Unless something re-flattens it automatically, mail from the provider's new servers starts failing SPF. Before flattening, remove unused includes, and consider moving bulk or marketing mail to a subdomain with its own SPF record.
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