I assume badly misconfigured servers meant something like open relay setup. Not DMARC and dependent technologies, which are not protection against spam at all,

I assume badly misconfigured servers meant something like open relay setup. Not DMARC and dependent technologies, which are not protection against spam at all, anyway. They are protection against various kinds of spoofing. And I concur with the original commenter. I do zero filtering on my SMTP server (no reverse DNS lookups, no blacklists, no nothing), in order to be sure I receive every single email, and SPAM looks like it would not be an issue even if I disabled the bayes classification client side, these days. I mostly rely on providing unique random addresses to each service. My oldest 18 year old address gets quite a bit of spam so I blocked it and switched to different one, but newer addresses that I use for the last 5 years didn't accumulate much spam, despite being public and crawlable on the web.