SPF record checker
Look up and parse a domain's SPF record - lookup count, the all-mechanism, and common warnings.
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What SPF does
Sender Policy Framework lets a domain publish, in a TXT record, which mail servers are allowed to send email claiming to be from it. A receiving server checks the connecting server's IP against that list - if it is not on it, the message fails SPF, which factors into whether it is delivered, flagged, or rejected.
The 10-lookup limit
Mechanisms like include:, a:, mx: and redirect= each cost a DNS lookup to evaluate, and RFC 7208 caps SPF evaluation at 10 of them in total, including nested includes. Go past it and many receivers return a permanent error and treat the record as if it failed outright - a domain can have a syntactically perfect SPF record that stops working purely because it grew too many includes over time. This checker counts them for you.
The "all" mechanism
This is the catch-all at the end of the record, and it decides what happens to a sender that matched nothing else: -all (hard fail, reject), ~all (soft fail, usually accepted but flagged), or +all (pass everyone - almost never intended, since it defeats the point of publishing SPF at all).
Related tools
SPF is one of three checks - see DKIM and DMARC for the others, and MX lookup for where the mail itself is delivered.
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