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What is SPF flattening?
Replacing the includes in an SPF record with the IP ranges they resolve to, to stay under the 10 lookup limit.
How it works in practice
Flattening trades lookups for length and freshness. The record gets longer, sometimes beyond what fits in one DNS response, so it has to be split into sub-records, and it only stays correct while the providers' IPs stay the same. It works best for senders whose IPs rarely change and badly for large providers that rotate addresses.
The common mistake
Publishing a flattened record once and never refreshing it.
Check it yourself
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