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It may not be "how phishing is determined," but failing authentication should be a quick means of determining that something isn't right.
Authentication is ok, other data is more timely and better S:N, getting any large corporation properly authenticated is very difficult, and cousin (look-alike domains) autheticate perfectly well. Spamtraps, user complaints, logs (web and mail), passive DNS and SSL Cert logs 1/2
2/2 there is no shortage of telemetry that is as-good, or better. What authetication does is offer a positive affirmation that mail really is from a given source. That’s good too.
Yes, but as a RECIPIENT, I don't get access to that telemetry. What I do have are headers that say things like "softfail" when authentication is checked. What I can reasonably check will sometimes change depending on what I'm doing.
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