Bartłomiej Piotrowski commented:
We are using RBLs AND greylisting AND Bayesian spam filtering, and all three reject significant number of junk messages. Despite of this, we still have a problem with multiple mailing lists being heavily spammed. I'll be playing around with rspamd but RBLs are not going anywhere; of these three, greylisting is the least useful method, spammers use well configured mail servers that correctly re-send messages.
Things are easier when it's not gnome.org but it is what it is.