Look up and parse a domain's SPF record online — see the policy and includes.
Find a domain's SPF record and parse it: see the mechanisms, the includes and the all-policy (fail, softfail, neutral). SPF tells receiving servers which hosts may send mail for the domain.
-all (fail) tells receivers to reject mail from unlisted servers; ~all (softfail) accepts but marks it. -all is the stricter, safer policy.
Misconfigured SPF causes legitimate mail to land in spam and leaves your domain open to spoofing.