I concur on this for the most part because I would say that my custom postfix + spamassassin + opendkim setup, on my self run MX, correctly classifies 75%+ of t

I concur on this for the most part because I would say that my custom postfix + spamassassin + opendkim setup, on my self run MX, correctly classifies 75%+ of the spam or outright rejects it for SMTP transfer just based on: a) invalid rdns of other mx b) invalid spf c) invalid DKIM / no DKIM signature d) failed RBL list check - I subscribe to and feed it a few different common sense SMTP RBLs Rejecting as spam things in the above category before it even looks at the content. Adding a high score for invalid rdns, spf or dkim before something generally similar to spamassassin or a more advanced message subject line/body analyzing system begins classifying things help. And then additional score is added of course for text spam content in message subject line and body.