"Domain of <sender> designates <IP> as permitted sender" — SPF Explained
"Domain of X designates Y as permitted sender" is the standard SPF-pass message embedded in email headers. Walk through what it means, how SPF works under the hood (TXT lookup, mechanism evaluation, result), the per-status-code meanings (pass, softfail, fail, neutral, none, temperror, permerror), and how to configure SPF correctly for common scenarios.