SPF Checker
Find and inspect the SPF policy published for any domain.
What is an SPF record?
SPF (Sender Policy Framework, RFC 7208) lists which servers are allowed to send mail using your domain in the envelope sender. It is published as a TXT record on the domain itself, beginning with "v=spf1".
A domain must publish at most one SPF record. Two SPF TXT records is a configuration error, and receivers are expected to treat it as permanently failing rather than picking one.
Reading the mechanisms
Mechanisms are evaluated left to right. "ip4:" and "ip6:" authorise addresses directly, "a" and "mx" authorise the domain’s own address and mail-exchanger hosts, and "include:" pulls in another domain’s policy — which is how hosted mail providers are typically authorised.
The final term sets what happens when nothing matched. "-all" is a hard fail, "~all" is a soft fail that asks receivers to accept but mark the message, and "?all" expresses no opinion at all.
The ten-lookup limit
SPF evaluation is capped at ten DNS-querying mechanisms — include, a, mx, ptr, exists and redirect all count. Exceeding it produces a permanent error, and receivers are expected to treat the result as unusable rather than ignoring the limit.
Long chains of nested "include:" terms are the usual cause. Each provider you add pulls in its own includes, so the count grows faster than the record looks.