@gumnos @stefano my setup is something that developed over a few years almost 20 years ago. Back then, exim was a reasonable MTA choice, I had been using spamassassin for a while when I set up a spamd in front of it.
The "18 years of greytrapping" piece https://nxdomain.no/~peter/eighteen_years_of_greytrapping.html has *all* the links, most of them potentially useful I think.
The main takeaway is that a greylisting spamd takes a lot of load off any content filtering (and then there is greytrapping for entertainment)