SPF Check — Who Is Authorised to Send as Your Domain
SPF publishes, in DNS, the list of servers permitted to send mail for your domain. A checker fetches that record, parses it, expands every include it references, and reports whether the result is valid — which is a more demanding question than whether the record exists, because SPF has hard limits that a syntactically fine record can still exceed.
Why run it
The ten-lookup limit is the reason. SPF permits at most ten DNS lookups while evaluating a record, and every include of a third-party sending platform consumes some of that budget — sometimes several, since those includes contain their own. Add enough senders and you cross the limit, at which point evaluation returns a permanent error and receiving servers may treat all your mail as unauthenticated. Nothing warns you. The record looks correct, and it is; it is just too expensive to evaluate. The other frequent finding is two SPF records where the specification allows exactly one, which invalidates both.
Why automate it
SPF records grow. Each new tool that sends on your behalf adds an include, and nobody removes the includes for tools you stopped using. That growth is what eventually crosses the lookup limit, and it happens one reasonable change at a time, usually made by someone who does not know the limit exists. A scheduled check that counts lookups and validates the whole expansion catches the change that pushed you over on the day it lands, rather than after a week of quietly failing authentication.
What it gives you
Verification that your SPF record is not just present but actually evaluable, and an alarm on the day it stops being. Given that the consequence is your legitimate mail being treated as unauthenticated, this is a cheap check with a large downside.