Validate an SPF include

Check that a domain is usable as an SPF include: and count the DNS lookups it costs, so you can tell before adding it whether it will push a record past the ten-lookup limit. The count includes the include itself. Callable with an API key or a dashboard session. The browser tool reaches this same endpoint with a captcha token instead, which is why you may see it called without credentials; a captcha is not something an API client can produce, so it is not part of this contract.