What is SPF 10 lookup limit?
The rule that an SPF check may trigger at most 10 DNS lookups, or it fails with a permerror.
How it works in practice
Each include, a, mx, ptr, exists and redirect term counts as a lookup, including those inside the records you include. A record that looks short can be over the limit once the includes of Google, Microsoft and a few sending tools are expanded. Ways out are removing unused senders, moving bulk senders to a subdomain, or flattening part of the record into IP ranges.
The common mistake
Flattening the whole record and forgetting it: the IPs are a snapshot, and when a provider changes them your mail starts failing SPF silently.
Check it yourself
Related terms
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